I use Eagle (from CadSoft). It's not open source,
but it's less
expensive than most commercial alternatives, and it seems to work
well. For nonprofit use, there's a "free" version that's limited
to 100x80 mm double-sided boards, or a US $125 version that can do
160x100 mm boards of up to four layers.
I also like Eagle. Too bad they charge twice that amount here in Europe....
I could use an upgrade to Eurocard format.
Jos