If you liked Thief you should get Thief 2. Then after you beat that, you can download around 400 or so player made Fan Missions for Thief and Thief 2.
Most are very small, but there are probably 20 or 30 very good missions. The largest ones have movie sequences, voice acting and huge leveles to explore. You might download some of the Thief 1 FMs and after you get through those, start on Thief 2 and the rest of the missions.
Darkloader is very good at keeping the save games and settings separate so you can play any of the missions very easily.
<A HREF="http://www.keepofmetalandgold.com" target="_new">http://www.keepofmetalandgold.com</A> Most of the fan missions and programs to run the missions. The largest ones have been taken off, but can be found other places. He has also rated some of them. I would start by downloading the ones with the highest ratings, and the largest file sizes.
<A HREF="http://www.thiefmissions.com" target="_new">http://www.thiefmissions.com</A> Has probably the same amount as the link above, including the large ones that have been removed, but only 1 can be downloaded each day, so make it count.
<A HREF="http://www.thief-thecircle.com" target="_new">http://www.thief-thecircle.com</A> Has links to the Thief Fan sites, and some downloads.
<A HREF="http://www.thief2x.com/default2.asp" target="_new">http://www.thief2x.com/default2.asp</A> The Home of the Thief 2X Project. They are working on a full 13 or so missions for Thief 2. 6 missions are ready for Beta testing, so it's moving right along.
<A HREF="http://www.blackcatgames.com" target="_new">http://www.blackcatgames.com</A> And this site has a Thief mod for Unreal Tournament. Play as a Thief or as a Guard, multiplayer deathmatch, using shadows to sneak around and become invisible just like the Thief games. They are also working on a Thief mod for UT 2003/2004 when it comes out. Very cool, since I always wanted Thief to have a multiplayer mode.