Just be careful when you use fonts from any site that says it's 'free.' Many times I've downloaded fonts, unzipped the files, and found the readme to contain something to the effect of:
"As far as we know this font is free for public domain use..."
If you use a font like this, you're app might become incredibly popular, someone sees their font in your app, and you get sued.
Unless it's very obvious a font is free for public AND commercial use (e.g., being said on the creators website itself), I would recommend to always take the extra step to contact the creator and get permission.