Block Frenzy

Block Frenzy: Click and hold the red square, then move it around so as not to touch the walls or the blue blocks.

Stratego Online

Interesting, so you can now play Stratego online.

Abstract Strategy Games

This wikipedia article on 'strategy games' has lots of interesting ones I've never heard of.

Kung Fu Chess

Wow, definitely one of the best chess variants I've played in a long time!

Pentago online

Pentago: Not as good as playing Five in a Row in your chemistry lab book with your lab partner when the teacher wasn't looking, but almost as good...

Owari Online

Play Awari online! (Mancala variants include Oware, Bao, Kalah, and Omweso)

Owari rules for newbies.