I've been having trouble finding a library to display code syntax like SO does. I'm really not 100% sure what keywords to be searching for. It needs to be able to handle any language (possibly by adding modules) and be free. 

I'd prefer libraries in either php or javascript/css as those are what my app is written in. 

Also, it would be awesome if there was an open-source project or something similar where I would be able to build on the code lexer.