Crafty is an xboard chess engine by Bob Hyatt -- featureful, strong-ish, based on a lot of theory and experience.
TogaII is a UCI chess engine by Fabien Letouzey, strong, but not tops.
Polyglot simulates an xboard chess engine by driving a UCI engine underneath; this makes shredder, stockfish, TogaII, Beowulf, etc, accessible in chessdb and xboard. This build of polyglot features additional functions useful for analysis.
ChessDB, derived from Scid, is a fine database program for analyzing existing chess games, or running an engine, but not for playing games.
I do not supply binaries, because you should not run binaries you find in random places around the internet. Compile them yourself.