I'm clearly biased on the matter, but here are a couple of options.
Two systems seem to attract most of the conversation around these boards. You can check each one's site to see if either fits your needs.
RTS (rts-solutions.com) is very popular, and can handle pretty much everything. It is inexpensive to get into, because all of the hardware is off-the-shelf. You can buy a system from them or build your own, depending on your budget and/or qualifications. Upgrades are frequent, they are responsive to customer needs and support is very good. The downside is that it is not a "buy-out" type of system. You pay a yearly license to run the software, depending on how many sales terminals you have. While there are pros and cons to everything, my opinion is that it's pretty hard to go wrong with this system.
Another system you might look into, is the "Sensible Cinema" (sensiblecinema.com) software. Like RTS, the hardware can be pretty much anything you want (PC-based). In this case however, the software is actually purchased. There is no annual fee to run the program. Support is free for an initial period. After that, anything other than emailed questions carries a charge. I also used this system for a while. It started out OK, but my needs were way over what they could/would provide at the time. My chief complaint was the availability of a live body to chat with, which always was one person (the owner). That was a few years ago, and they may be better now, so you should probably check it out.
As far as I've seen, everything else out there is more expensive, or is based on proprietary hardware designs that, once you've invested in them, keeps you tied to that particular company.
If you have a well-developed business model, a buyout program might work for you. On the other hand, if your business tends to evolve (like ours), a system that updates frequently might be a better choice.
Both systems have demos available. RTS can be downloaded from their site. It is fully functional, except that it won't actually conclude a sales transaction (must be licensed for that). For Sensible Cinema, you need to go to their site and request a demo. I don't know what limitations are on that.
Good luck!
[This message has been edited by rodeojack (edited January 02, 2008).]