Yoga, pilates, and cylcling have gone hand in hand with horses recently. Another thing that does wonders is trotting stirrupless. That will really give you a work out believe me. I did it once for 3 hours straight and I couldn't sit the next day I was so sore.
Squats are good too. And if you aren't into the big barbell fiasco never fear! Just grab some canned food around your house and hold those straight down and do squats. Oh and then while you are doing that just do Straight-leg Dead Lifts. Scary name, but not that bad. Just hold them w/ your arms down along your sides then bend over like you are touching your toes.
Wall sits work too, but I personally don't like them.
Another thing I do is ankle weights. Just to wear them around the house to make me a little bit stronger w/o really noticing.
I also have a thing called "Leg Magic" Just got it today, but I'll let you know how it works. It's a pretty inexpensive piece of exercise equipment.
Hope that helps