Quite a few out there to choose from, starting at FREE and going up in value from there just dependent on what features you need. Zencart is a free one, easy to use admin, but it's a very basic platform, as you would expect for paying nothing.
The cart can be skinned, as many can now, so you do have a certain amount of flexibility in regards to design. Of course you can always opt to get custom applications built for it as well if required.
It's probably a good idea to work out what it is you really need in a store, and then start looking for an option that will tick all the boxes.