Couple of useful links for this... The first is SourceForge, and a quick search on there for Point of Sale
reveals a fair number. And, while looking at the second item in that list, a suggested/related article was on slashdot, specifically somebody
asking about POS software for their business. The comments have a lot of answers.
If you try any of them out and find they work well, I'd love to know how they are, and which one you decided to go with in the end.