I used to do this, it's a great stretch, and I will always bring my knee up before I get outta bed from now on
. Another stretch is: lying on your belly, if you can get your heel to your butt, one leg at a time. This also will stretch the front of your thigh (quads).
Hold for no more than 15 seconds, 3 times a day (as recommended by a PT)
This motion is a great exercise, too. (also advised by a PT to me for 2-3 times a week).
At first I couldn't do many in my current state, but now I can do 10.
These days I do the "butterfly stretch" that I learned in PE at school:
Sit, heels together, and just lean forard a little for an inner thigh stretch.
I LOVED this stretch when I could do it: Sit with one leg curled and other leg stretched out behind you.
I learned this one from Deniese Austin's gentla yoga show that was on Lifetime in 2002 or so.
Another great program is "Sit and Be Fit" on PBS.
Check pbs for that or similar programs.