as some have made a note already, it really depends on how big your company is, what kind of features are you looking for, and most importantly - what kind of budget do you have for a new ATS.
Some very good suggestions have been given here, but of course it all depends on your situation. In case your company is not a huge enterprise and you look for something easy I'd encourage you to check out SmartRecruiters at www.smartrecruiters.com. It might not have all the bells and whistles of the big boys out there and it is still in Beta, but it is completely FREE of charge. No trials, restrictions, or limits. Free and easy.
After all, why do you have to pay for an ATS?
Good luck with your search!