Please find below a list of abdominal and waist (oblique) exercises. Nowadays, core strength (lower back & abdominals) is always stressed upon - with separate workout classes even dedicated to the matter. However, the reason to have a strong core is because it acts either as a mover (synergist) or stabilizer during most human movements. Therefore, the hardest work your abdominals will do in the gym will be to assist or stabilize your body during other lifts (not ab specific exercises), subsequently, do not train your abs more than any other body part and leave training abs till the end of a workout (so it's not to the detriment of another exercise).
Another common reason for mistakenly over training abs is the want of a six-pack. Your abs are already there! The only way you're going to achieve a visible six pack is by reducing your body fat percentage, which you'll need to do through diet and increased exercise - you cannot spot reduce fat (unless you have lipo)!
Remember, sensible free gym workout routines can be found on the gymprofessor.com website. Better-still, download our free workout app (GP Shuffle), which also features step-by-step exercise demonstration and, should the need arise, the ability to shuffle to an alternate exercise.
You should also read the Gym Professor article 'How to get Six-Pack Abs'.