Even More Thieving
Well, maybe you've finished the game a few times. Tried it in "expert mode" and made some experiments. And you want more.
again... and again...
There are playing styles and variants; I can offer you my way (the "Lytha way") here.
And why not setting yourself different goals for the missions? Say, arrange the guards in nice patterns and take screenshots of it, or sneak in unseen and eliminate one target (assassin style). Or whatever you can think of - the Thief game lets you do plenty many things.
A guild of thieves
If you want conversation with other Thief fans, try the TTLG forum. There are other forums, but this is the place the game's creators are in. :)
There's also an IRC channel, #thief on starchat.net (try polaris.starchat.net:6667).
If you want more Thief, not just while playing, well, there's something for you:
Fan made missions
There are a bunch of fan made missions, and the number is growing. Find out more:
roll your own
You want to create your own Thief missions? Great!
It sure is a lot of work to get it right, but give it a try:
Thief 2: The Metal Age is out! So if it is available at your shop, buy it and resume your sneaksie fun!
If you can't find it at a local store, you can order Thief 2 on the 'net.