I'm a physical therapist, so I know quite a few, but I always like to find new ones to try out. I chatted up my Google+ communities and got a couple good ideas as well.
Here are the hip flexor muscles:
The only one that is missing is the Rectus Femoris which is part of the quadriceps.
The Adductors do not flex the hip, they bring the legs medially.
So you have Psoas major & minor, Iliacus, Sartorius, and Rectus Femoris as your
major hip flexors.
These are a few exercises that I started with last night:
Straight leg raises (I did 2x10 for each leg of these.)
Bridged Knee Extension with Adductor Squeeze (I did 1x10 on each leg of these and I squeezed my foam roller.)
Ball Pass for Core & Hips (I did 10 total of these.)
I then proceeded to foam roll my quads (I only did 5 total of these, they were so sore!)
What are your ideas for some strengthening?