Google has launched a new search called Google Code Search in Google Labs for searching billions of lines of public source codes, this will help developers search the web for existing code already written by other developers.
Though this is definitely a boon for developers like me, earlier i used to use another such service provided by koders.com which helped me sift through existing code that i could make use of instead of re-writing the whole code.
You can use Google Code Search to search using regular expressions, using exact strings, search for files matching regular expression, search for packages that match regular expressions, search for code written in particular languages only, search for code distributed under certain licenses only.
Overall its looking good and will be quite helpful to developers like me too.
You can try out the Google Code Search.