Leg raises are BY FAR one of, if not the, best AB exercise you can do.
How to: Either hang from a pull-up bar or use an ab station (Which I have and like better). Extend your legs straight down and lift them up parallel to your body.
If you can't quite do this (I couldn't when I started) do knee ups instead where you keep your knees bent and lift them to your chest.
AB Station:
[img][800:550]http://fittipdaily.com/wp-content/uploads/2012/11/straight-leg-raises1.png[/img]
Planks are also another fantastic AB workout.
How to: Hold yourself up parallel to the floor, keeping your back straight, and resting on your toes and forearms. Hold this position for as long as possible! I'll typically hold for 1.5-2 minutes and repeat. Like the picture below:

Great exercises, also you can do reverse crunches. Even I do pullups I feel it in my abs. Any kind of workout you do standing will also work the abs to some degree, since they're stabilizer muscle in pretty much anything.