I am using a generic silicon case that I got on ebay it wasn't one of the 1 or 2 dollar cases I think I paid about 7 w free shipping. Its black I like it bc the focus wont fall out of my shirt pocket My wife doesnt like it bc after time it starts to pull the pocket on my dress shirt's out (wear dress shirts 6 days a week generally).
I just got a Body Glove Mirage case from Ebay for 5 bucks shipped. Works great, keeps the lighter weight of the Focus and doesn't add much thickness. I've also got the red Pop case by Casemate - its good too but after a while the thickness was starting to bother me.