Personally, I prefer offline tools: I don't see the point of parsing the code (particularly large ones) over and over, for each served page, or even worse, on each browser (for JS libraries), because as pointed above, these libraries often lag (you often see raw source before it is formatted).
There are a number of tools to do this job, some pointed above. I just use the export feature of my favorite editor (SciTE) because it just respects the choices of color I carefully set up... :-) And it can output XML, PDF, RTF and LaTeX too.
