If you do a search for "free wordpress themes" you'll find thousands of options but, in my experience, the really beautiful themes are hidden gems offered as freebies from designer's websites, or their online portfolios.
Of course, any theme that's free means there are no guarantees regarding functionality, ongoing updates, etc. But a bit of research will usually turn up some helpful user reviews so it's a good idea to suss that out.
1. Creative By Nature | Author: css-mayo
Not too sure what the chocolate chip cookie is all about, but I like how layered and textured this design looks.
2. ChocoTheme | Author: css-mayo
You have to see this one up close to appreciate it (follow the link above to do that). I like it because of the 'leather bound diary' look.
3. Desktop Chaos | Author: Evan Eckard
Evan Eckard has designed some beautiful themes, and he has a whole gallery of freebies!
4. Notepad Chaos | Author: Evan Eckard
This is my favourite! I found it after I'd setup my site with its current theme and still can't stop thinking about re-doing the whole thing. Hmm...
5. Zexee | Author: Six Revisions
Here's another one that looks better up close. Follow the link to see. I love anything involving the colour teal, and it works really well here with the deep red. A bold design move, and it works!