I'm basically with Grant on this one. If I know the source and I know that they are conscientious about testing I'll usually use it, although even then I usually run it against a test/dev system first to make sure I understand it.

I can't find the post now, but Buck Woody had a blog post about being sure to understand and test any scripts/code you get online before using it in production. If/when I find it I'll post it.