Whether you're an ajax, php, perl, python, sql, mysql, or even ruby on rails (ror), these code search engines can come in mighty handy.

While these search engines handle much more code than mentioned, each one may have different amounts of language available to find code in.

These are listed in the order I found them:
1. http://www.koders.com/ (wide array of programming languages)
2. http://www.google.com/codesearch (handles illegal characters well)
3. http://labs.oreilly.com/code/
4. http://www.codase.com/ (java, C++, and C only)
5. http://px.sklar.com/ (PHP)

See also http://en.wikipedia.org/wiki/Open_source#See_also (opens new window or tab)