Uh, yeah, public domain means it's free to use. That's how you get all those public domain ebooks for free at Project Gutenberg or this site.
I'd be careful with translations, though; if you use a 21st century retranslation of a Brothers Grimm fairy tale, while the tale itself is definitely public domain, the translation probably isn't. (I don't know 100% for certain, but I'd rather be safe than sorry.)