I came across this interesting and entertaining article the other day called “Why You Should Never Search For Free WordPress Themes”
It analyses a bunch of free themes from various places and finds an astonishing amount of hidden, encrypted and plain dodgy code placed with ill intent.
Sometimes it is best not to accept things from strangers, just like Mama said. It’s a jungle out there and often if something sounds too good to be true, it probably is. (Add your own proverbial saying here.)
But don’t take my word for it, Have a read.
The article also links to another article on places where you may be able to find good free themes.
Related article: The Thing about Free Stuff: WordPress, Themes & Plugins