An Ab wheel can be used to concentrate on the core area.
A Plank/AB Exercise Pad can be used to provide comfort to your arms while performing front and side planks.
Pull Up Ab Straps to be used with a pull up bar to do knee ups, leg lifts, and oblique twist.
TRX Suspension Straps to isolate the core and hit from different angles.
Medicine Ball exercises.
BOSU Ball can be used to add balance to exercises.
Stability Ball can also be used to add balance and difficulty to exercises.
Resistance bands and tubing can be used to provide resistance to strengthen the core.