If you're a retailer, looking for a POS system, you've probably found several open source POS systems. These might have intrigued you because they're supposed to be free. POS systems are expensive, after all, and you shouldn't pay thousands of dollars if you don't have to. That wouldn't be very smart.
It might be possible to get a free POS system by leveraging open source software, but there are some things you must keep in mind. First of all, open source means that the source code for the software is available. It doesn't necessarily mean that someone is going to hand you a complete POS system for free.
A complete POS system includes at least software, hardware, setup, training, and support. You'll probably be able to get the software for free, but you may not be able to get the rest for free.
Now, if you're lucky, you'll have POS hardware in your store already and the new open source system will work with this hardware. This is something you'll have to investigate.
If you don't have any hardware yet, you'll obviously have to buy it. If you look around, though, you may be able to buy it second-hand for cheap. Retail stores are going under all the time after all.
You may be able to get some or all of the other services for free as well. Most open source projects have information about setting up and using the software available via a website or wiki. The quality of this information varies depending on the open source project. How much information you need will depend on how technical you and your staff are.
These projects usually offer support via a community forum. When you have questions, you post there and other users will answer you. These forums may or may not be helpful. You can usually sign up and ask questions before you start using the software. This is probably a very good idea. If you want personalized support via email or phone, though, you'll probably have to pay somebody for a support contract. It's doubtful anyone will offer 24/7 support like this for free.
Finally, unless the system happens to come with all the functionality you need, you'll need to pay someone to customize it for you. This can be very expensive. Because the software is open source, though, at least this is a possibility. This isn't really a big deal, though, because you'd have to pay someone to customize any POS system. No one is going to do this for free unless a lot of people are asking for the same functionality. The developers may add it into the next version of the system if this is the case.
Basically, an open source system may work for you, but do your research ahead of time. Make sure the system is going to work for you, and that you'll be able to keep it running, before you start using it in your store.