I have a good stretch for the hips & groin area, but it's too hard to explain. I'm waiting for a friend who knows her yoga well to email me back with its proper name so I can google for a picture.
Edit: Okay, it's called "pigeon" (don't ask me why).
Your back leg is
straight back, you can see how the front leg is, and you're pushing the bottom of your pelvis down into the mat. It stretches the top of your back leg really nicely. Keep your torso up straight, and don't over-arch your back. After a 30 count, switch legs.
The variation of this is to do the same pose but drape your upper body over the front leg so you're resting your head on your forearms. Now you're getting a stretch of your glute too. Again, after 30, switch legs.
I've been doing this one regularly for quite a while, and it's definitely made me more flexible towards doing the splits.