Fritz: Tactical, but positionally sound. Well rounded.
Hiarcs: Positional and human-like play. Good in unbalanced positions.
Junior: Very tactical, sacrificial style play.
Rybka: Strongest chess engine. Excellent positional understanding and human-like play. Excellent evaluation of dynamic positions. Great for analysis.
Shredder: Very positional and solid. Excellent endgame play.
Zappa: Human-like play, aggressive. Very strong, catching up to Rybka.
Fruit: Well balanced positional play.