You can use any font you like — as long as the font’s license allows you to!
Wordmark logos, which consist only of text, are an exception to this rule. In these designs, you must provide your own, original font. It can be based on another font, as long as the font’s license allows designers to make derivations from it. Additionally, adding a box or line to an unmodified font is still considered a wordmark and the font will need to be modified.
The font’s licensing information and terms of use should be easy to locate wherever the file is available — provided it’s from a legitimate font foundry. If you can’t find the license for a font, don’t use it.
You should always check the licenses for use of any fonts you didn’t create yourself.
If you’re using an unmodified commercial font in a logo, illustration or button/icon contest, provide it in outline format, so you don’t have to provide the font itself. If the client requires the font files, they’ll need to purchase the font themselves.