Battle for Wesnoth is a great TBS game for the PC with loads of RPG aspects. Best of all, it's open source, free, independently developed, comes with a map editor, and has fully-functional online play. It's really a great game, set in a midieval fantasy world. The game is always being improved, with regular updates from the community as well. I've been kicking it about for a while now, so I'll share it, and hopefully we can play some in the near future.
http://www.wesnoth.org/
(1.0.2 Stable is the most current version to download)
Some cool screens:
(Note: The game just went through a big update to 1.0.2, and some of the art has changed since those screenshots were taken.)