For
openFIRST we currently use CVS but are in the process of migrating to Subversion (it was only last week that
SourceForge rolled out Subversion support to all projects).
We're running this in an environment where contributors are (or have at various points been) spread across at least 3 different countries (and two continents). They both work well over dialup and/or unreliable connections (well, as well as anything can work over an unreliable connection).
For learning either the SourceForge site documents for
CVS and
Subversion are good. They've also got
recommended software for both, too (Subversion is below the CVS one in that list).