Gametap is about £6 a month to join, as long as you are a member you can download and play any of the games on there. However, it is built for the US and canada and anyone in those countries should not be able to even register!!
Problem is, they are more than willing to take your money from over here but many of the games have restrictions placed upon them that states they can only be downloaded and played in the US or Canada. Psychonauts was one of those games (2 months ago).
Not sure if it has changed but if anyone still wants to play it then I came up with a way around it. Simple go into your account and change the country to the United States. Very simple but will work.
As Flammy mentioned, its free but it is time based. The older games are left on there for ages but the newer/popular games are only on for about a week!
The free client does indeed use adverts, however, it is only when you initially load the game. Once it is up and running you won't see any.