As demonstrated by the dosh that they command these days:
The rise of open source software in application development puts developers with a specialization in those technologies in a position to ask for a 30 or 40 percent pay increase, Kirven says. "We've gotten more requests from our permanent placement division for open source developers in the last six months than in the last five or six years combined," he says. "It's not as easy as getting free software, someone has to get it up and running. LAMP is everywhere now -- these types of technologies no one heard of 18 months ago are all the sudden becoming a hot commodity."
Admittedly, this in the US: anyone with experience in the UK or Europe?