OSS point-of-sale systems?

Hi all,

Since we had such good luck with the Vonage question, I figured I'd try
another somewhat off-topic question. ;) A friend of a friend back in
Iowa is currently running a small retail store off a decrepit, DOS-based
point-of-sale system which is no longer supported by its
(out-of-business) manufacturer. The store owner would like to move to
Windows XP (yeah, yeah, I know...) and is looking for a well-supported
replacement for his POS system.

SourceForge lists a number of possibilities, but I don't have much
information beyond that. Does anyone here have any experience with an
open-source POS system (perhaps backed by Postgres?) that they can
recommend?

Cheers,
--mlp