The nice folks at O'Reilly, who make very good geeky books, have a fun little quiz going on. They show you a code snippet and you guess what book it's from. This is usually (but not always) a matter of guessing what the language is, and that's harder than you might think at first. There's a little timer counting down so the longer you take to make up your mind, the less points you get. Here's a really easy example:
<p>Here is a paragraph.
<p>And here is another.
Is that from:
- Gaming Hacks
- Learning XML, 2nd Edition
- GNU Emacs Pocket Reference
- HTML & XHTML: The Definitive Guide, 4th Edition
Trust me, most of them are much harder