Sit-ups, high plank, low plank, side plank. Crunches, reverse crunches, bicycle crunches, side crunches, oblique crunches. Mountain climbers, knees to shoulder, leg lifts, scissor kicks, bicycle kicks, flutter kicks, jackknives, in-out abs, ski abs, the C-sit for the V-in.
The amount of painful, unpleasant shit people [N.B. myself incl.] go through to work out their abs, when the best ab exercises are simply laughing and fucking.