I try to maintain an up to date(ish) list of how much individual ISPs are charging and any caps or FUPs that are in place, the details of which can be found on my ISP caps and prices page. In the past I've always maintained a small "rant" section on the site of ISPs who advertise unlimited, yet do in fact have some form of FUP/SUP/AUP.

At the time of writing this (Feb 07), it now seems common practice for many ISPs to falsely advertise "Unlimited", yet have some form of Fair Usage Policy in Place. Its pretty well known that within the ISP industry that no IPStream ISP can afford to offer unlimited bandwidth due to the high costs of BT Wholesale Central Pipes.

ISPs currently advertising Unlimited.

The following ISPs are all currently either advertising "Unlimited" products which do in fact have a FUP, or have a FUP which doesn't advise you how much you can use:

AOL, BT, Demon, Eclipse, Orange, Pipex, Supanet, TalkTalk, Tiscali, Toucan, UKOnline, Virgin.

The degree to which the FUP is hidden or undisclosed varies but some of the above are more unfair than others.
ISPs get around this by saying things like "No need to monitor your usage", but what in fact they are doing is saying, Yes you can download what you like but download too much and we will throttle your speeds. Common throttles are 256 or 128 kb, which on adsl makes downloading anything nigh on impossible. Even surfing can become extremely slow at these speeds, possibly due to ATM overheads and dropped packets, which makes things appear slower than dialup.
Another common practice is to slow speeds down for all users during peak times.

This practice is unfair for several reasons:

• Hidden FUPs mean that users aren't easily able to compare packages across varies ISPs to find out which may be best for their own circumstances.
• Unfair on Users. There have been many users who have signed up to what they thought were unlimited packages, only to soon find that their accounts have been heavily throttled. By which time its too late and they are locked into a 12 or 18 month contract.
• Unfair on those ISPs who do try and lay out clear CAPs and FUPs. Since it puts them at an unfair disadvantage from selling their products in a market where many ISPs who are hiding FUPs. Too many users simply see the word unlimited and believe it.

Be aware.

No ISP can afford the huge costs of true unlimited bandwidth. It costs them millions of pounds to install central pipes to provide more bandwidth which would be required to ensure that the higher speed products can get truly unlimited. As a rough guide if you see unlimited 8Mb for less than £80 then check carefully. If they are offering 8Mb unlimited for less than £40, then the odds are there will be a FUP in place, less than £20 and there most certainly will be.

Historical Info.

Blatant abuse of the word "Unlimited" awards go to the following ISPs, who do actually apply some sort of capping/limits despite advertising otherwise.

Homecall

Advertise "Uncapped 2Mb Broadband".
Careful inspection of their T+C's - Section 4.5 reveal that an AUP policy is in place and the limits aren't defined. Download too much and expect speeds of 28k/B.

"I have been with homecall for 4 days. I signed up with homecall 2MB service due to their attractive "UNCAPPED BROADBAND" and "UNLIMITED DOWNLOADS" offer as stated on their main page, only to find my connection cut to a 28kps connection, having contacted homecall, i found out that they have a user share ration limit - this limit is not documented anywhere and no warning about reaching "THE LIMIT" was issued. they refuse to terminate the contract or re-instate me to a 2MB connection for a minimum of 3 weeks and that's if i don't use any bandwidth."
Source adsl-guide forums 07-Oct-05

Supanet

In the past Supanet have blatantly advertised no caps yet there have been many reports of users speeds being throttled to unacceptable speeds. P2P ports are blocked/throttled - 119 and reports of bad speeds are rife.

Update Oct 05. Supanet have now introduced accounts with fixed caps and one account advertised as Unlimited* - note the asterisk. I cant find anything on their site regarding a FUP, but in view of past history I would still be cautious.

TalkTalk

Advertise "Unbelievable unlimited 1Mb Broadband".

Don't believe it, they certainly haven't defined any limits, but for many months they have been cutting off some of their users without warning. The small print T+C's 4.5 state:- "we may change your Transmission Speed or manage your use of our Broadband Service as we see fit in the circumstances."

During 2005, the limit for disconnecting users was assumed to be <30GB and throttling occurred on figures much less than this. Some users were getting dial up speeds and unable to use the connection for gaming.
Update Oct 05 - Evidence from users which would imply that traffic shaping and port blocking appears to continue.
Update Dec 05 - Its believed that TT are now port blocking all p2p traffic during peak time hours.
Update Feb 06 - dea-tor kindly advises me information on how "heavy users" are restricted:-
        09:00 am until 01:00 am (16hrs total bann on P2P/bitorrent)
        01:00 am until 09:00 am (8hrs up to 50% throttling)
Unfortunately what they maintain as a heavy user has still not been clarified. :/
Update Apr 07 - Users are now reporting p2p blocked between 8am to midnight, and p2p speeds throttled even outside these hours.

Tiscali

Blatantly advertising "New Unlimited Packages" across all media. However careful inspection reveals that a FUP is in place. In view of Tiscali's past history users have been throttled to poor speeds when they have used amounts between 15-30GB. It would appear from the FUP that users are encouraged to schedule downloads to off peak periods to avoid throttling.

Update Dec 05 The only guidance I can seem to find from Tiscali is

"Usage during peak times is limited and measured over a 10 day period. While I cannot offer exact figures, I would strongly advise that you do not use P2P etc during peak times for large file downloads."

Users that have been caught out by the "unlimited" advertising have found themselves on speeds worse than dial up. Other reports are that Tiscali are blocking all ports except for port 80 (surfing).

Its not the first time that tiscali have done something similar.
ADSLguide News Archive

Toucan Broadband.

Tiscali resellers, so it's extremely unlikely that they will be offering unlimited despite their advertising.
The asterisk points to a Fair Usage Policy which states that "we may restrict over-use by heavy users by, for example prioritising traffic from light users during peak periods".
It has recently been confirmed that Toucan consider 3GB per week (12GB per month as being) "unlimited" :(

Since day one (29th March) I've been experiencing terrible speeds during the day and good speeds after midnight so I telephoned to find out what was going on and clear my suspicions that they were maybe capping my service, after a major whinge I was informed that there were in fact limits.. 3GB per week!!!!! which is quite clearly not the unlimited service they are so blatantly advertising. There is also no mention of a 3gb limit in the fair usage policy. I was also politely informed that I will receive up to 5 emails warning me to curtail my downloading before they will disconnect me.. Unfortunately I'm now tied into a 12 month and my only way out is to download like crazy so they disconnect me,, no option to obtain a MAC code even, which will leave me without any internet. Kind of wondering why I bothered,, I've been mis-sold a product
[source adsl-guide 01-Apr-06 ]

Wanadoo/Orange

New one to add to my list of falsely advertising as unlimited. Which is surprising since they have only just been found guilty of false advertising for their "8Mb product" (details). You would have thought they would have learnt a lesson.
The word unlimited* is asterisked which points to their FUP which states:-

Some of our broadband customers use file sharing software and download large files like music and videos. This uses up lots of network capacity leaving less available for you. If they’re doing this at peak times, it could mean that the speed of your broadband service will be affected.

This would seem to indicate that if you p2p then your speeds could be throttled?
Latest information is the amount they consider to be "unlimited" is 30GB or 40GB for the old FreeServe accounts .

The bad news is that the new "uncapped" service is indeed still restricted to approx 40gb per month, the only difference is that the new packages may allow us to switch to a higher speed of service

Considering an awful lot of customers wont be able to get the increased speeds, not much change really :(