If you want to try and hit multiple planes and movements...
Russian Twists with medball
Supermans (with or without medball)
Toe Touches (aka V-ups), again with or without medball
Ab wheel rollouts (you can get a wheel for a few bucks)
In terms of isometrics:
Planks of all variations
L-sits ( http://www.crossfit.com/mt-archive2/L-sit.html )
Pullup bar hang with knees to chest
But one of the most underrated and underused methods to build insane core strength and stability is by lifting heavy weights. Deadlifts, for example, require a ton of lower back strength to keep your from folding in half (same with squats). It's hard to find a non-weighted exercise that provides similar bang for the buck.