I think the standard one at Universities is turnitin. The Uni should offer a version which doesn't remember, so your thesis won't come up as all being plagiarised.
I use google. Simply copy and paste the sentences that look too good for the student level and you'd be surprized!... Last year I taught a course and based on the guide line 30% of final grade was based on the students research projects. They had to write a tech report. One of the students simply copied and pasted his report from few journal papers and only changed few headlines to match his project title. I randomly picked few whole sentences that were too good for the class level and googled the whole sentences and voila! They all showed up! Needless to say he was reported and failed the course.
KimWipes - I've done that too and found whole essays copied from (usually earlier edition?) textbooks which are available online. Makes me wonder how I got away with it when I was an undergrad haha /joke.
In other news, I note that Hiini hasn't actually said *why* s/he needs the plagarism detector? Always interested in numbnuts getting busted because they think (older!) people are daft :D