At the Bicycle Kitchen in Los Angeles we've been using Lightspeed (previously known as MerchantOS) for a couple of years.
The software was originally developed for a bike shop so they have some integration with various bike vendors. It works well enough, and it's nice to have something accessible over the internet.
Also, as a non-profit, it's free for us,
http://www.lightspeedretail.com/cloud/blog/2011/12/doing-what-we-can/