TLDR version: Tamron 75-300mm f/4-5.6, Canon EF 75-300mm f/4-5.6 III, Tamron 70-300mm f/4-5.6, or Sigma 50-200mm f/4-5.6 for equine photography?
I recently added the nifty fifty, and I'm enjoying the hell out of it - a lens can really change how you look at certain shots.
Anyways, all I have right now is the 18-55mm kit lens and the 50mm 1.8. My fiancee is an equestrian, and its obvious to me 55mm is in no way appropriate for getting shots of her riding. What I'd like to do is get some snaps of her riding, and maybe use my cam for a new video of her riding (the last one I took being from a few years back).
Since I have a wedding to pay for, I'm being cheap, thus the sub-$200 lenses.