I use four checkers for every submission I receive:
1. Duplichecker.com - absolutely the best, IMHO. Free, 25 checks a day with registration (free), spots almost everything.
2. PaperRater.com - getting a LOT of false positives, but it's a good spotter. Unlimited use (as long as they're not flooded with requests) and very fast.
3. SmallSEOTools - captcha every time (ugh) but not bad. Also free.