This pair of exercises – particularly the Hanging Knee Raise – are high on the favourites list. Both of them fall into the category of ‘ab exercise with additional benefits‘; largely in the form of some grip work.
Hanging leg raise
Hang from a chin-up bar (I usually use a medium-width, pronated grip for these). Bending at the hips, raise your feet toward the bar; keeping your legs straight. Perform this as slowly and strictly as possible – don’t use momentum to swing your legs up.
Hanging knee raise
The only difference in performing these is that your legs are bent throughout the movement. This is slightly easier, and if you can’t do the straight leg raise, try these. Much better than crunches.
A few things to remember:

• Try to keep your arms locked throughout the exercise.
• Squeeze the bar tight – this will help kick your lats into motion.
• In the straight leg raise, stretch your legs out. This will help make the exercise stronger and safer.
• Push your thighs together – this will help ensure a tighter contraction of your abs.

I generally perform this as a high-rep warmup, finisher or as part of a feeder workout. It seems to nicely stretch the spine after a bit of deadlifting.