From the same chaps who’ve recently launched Disco, that smokin’ disc-burning app, comes Checkout. This is a nice looking and well-featured point-of-sale application, including stock and order management. Having used a few of these myself, this seems pretty much sorted from what I can derive from the screenshots/screencasts on the site. It’s really for the smaller businesses at the moment though as you can’t network copies together, so you’d be reliant on one till, however it is ideal for small online stores where all your transactions are postal and can be handled by a single computer.
Checkout Retail Manager
This looks like a really old post (I mean, it's from 2006 for goodness sake). Stay for the retro vibes but be aware any information in this post is likely way out of date.