On the floor they're fantastic. They're hard at first. Go up on the balls of your hands to get some extra height, and then try just lifting one leg off the ground and holding it up, then the other leg. Build up to both.
Check out the beast skills site i linked to earlier. That has a couple other great ways to progress to an L-sit.
Other great exercises for the core (because you want to work the whole core, not just the front. Interesting note: It's actually the TVA or transverse abdominus (the muscles on the side of the stomach) that will give you a flat stomach and six pack, once all the excess fat is burned away. Situps don't work these at all, obviously.) is quadrupedal movement, or QM. Done correctly, this is an absolutely fatnastic workout.
http://www.youtube.com/watch?v=EQvkynl5GFI Ignore the jumping stuff he does, that's the only example I could find on Youtube. To make it even harder, spread out. Keep your hips down lower, and parallel to your shoulders. Another way to make it harder is to do a pushup after every "step" (move your left hand and right foot, then pushup, then right hand and left foot, pushup, etc).
Another exercise is the plank. Go into a pushup position, then go down on your elbows. Then just hang out and stay there. Keep your neck, back, and legs in a straight line and use your core to maintain the rigidness.
There are so many great core exercises. Just do situps? They're fairly useless in terms of how LITTLE of the core they work.