There are a *lot* of sites on our servers that use oscommerce:
The reason a lot of them look 'the same' is because the webmasters haven't taken the time to fully customize it. There's a *ton* of hacks/mods for it and it's easily customizable. It's also a nice program, overall, and the price can't be beat (free). Check out some of the example sites here:
2nd most popular is xcart - don't have as much 1st hand experience though with this one.