What are some effective abdominal excercises?

Incline crunches.

Sex.

Forced farting.

bicycle kicks

Laying on your back and making bicycle motions with your feet in the air. I do 300 of these three times a week.

Situps with rotation. 1. Lay on your back on the floor. 2. lift you feet 6 inches off the floor and hold them straight out. 3. rotate as you sit up bringing your left elbow to your right knee. 4. repeat on the other side, bringing your right elbow to your left knee.
Like this but have your legs out straight and bring one up as you sit up

Oh and @azlotto, crunches were proved to not be as effective as previously thought, this is because they only work a small section of your abdominal muscles. With proper sit ups you get the full rotation working all your abdominals and obliques

Core training is a key part of effective abdominal exercises. Being on the upside and downside of a scale throughout my life due to genetics and having a child has taught me the challenges of getting a fit body. After gaining 65 pounds and a predisposition to a pear shaped body, I found that core training was the boost my abs needed to really start to take shape.

Practice plank, and side plank, they may take a while to work up to a minute +. You can modify these movements as a beginner.

Stability exercises like a pikes on a ball will really hit areas other ab exercises can’t. This exercise involves being face down in push-up position on a stability ball with only your shins on the ball while trying to pull your knees in toward your chest. An advanced version of this exercise involves you trying to get your body in a upside-down “v” on the ball as well.
http://pilates.about.com/od/pilatesontheball/ht/Pike-Ball.htm

One really tough one that I truly despise (that means it works!) is lying scissors. The objective is to lie on your back with neck and head supported yet slightly off the ground. Lift your legs off the ground – the closer they are to the ground, the harder the exercise. Attempt to make a scissor motion with your legs out and in – parallel to the ground – while lifted off the floor. This is excellent for lower abs. There is a similar pilates move, this is not to be confused with that exercise.

Reverse crunches on the floor or a bench will also help to target the upper and lower abdominals. Perform these by lying on your back. Hold on to the top of the bench behind your head and try to bring your legs into a 90 degree position while keeping them straight.

For added challenge, you can press your feet towards the ceiling once in the “L” shape., or use a decline or incline bench.
http://www.nowloss.com/the-best-exercise-for-six-pack-abs.htm

The trick is that you may not be able to do some of these exercises more than 5 times the first try. If you keep at it, and work towards 15–20 reps for 3 sets, 3 x a week, you will definitely find these to be effective.

The key is to continue to confuse your body. Anything that you repeat for more than a couple of weeks in a row while be easily adjusted to. Find a variety of exercises that you can rotate, and be consistent with, and you will find your effective ab exercises.