Raillery: the funny underbelly of computer games
There's a bunch of tile sets available through Bay 12. I don't know if they cross versions.
This is one of the most comprehensive and prettiest tilesets I've seen and it's the one I use. The download has it pre-installed to DF (0.31.06, the current version), so all you have to do is download it and start playing.http://mayday.w.staszic.waw.pl/df.php