Are there any good e-commerce extensions/plugins for Umbraco?
The only other package I'm aware of is umbraco commerce
It doesnt look like it's been updated in a while and Commerce4Umbraco is nearing first release so is probaby a better bet.
I know that it will be featured at the Codegarden09 umbraco conference.
As Umbraco is .NET based you could also use an off the shelf shopping cart, such as dotNetCart or develop sometihing bespoke. Of course this would mean separate backends, but again you could easily fudge it into the backend end if required.
uCommerce is a fully integrated e-commerce package for Umbraco 4.0. You can see a full feature list at the uCommerce website.
It free to use for developers and you can download a copy from our website. .
Putting this here for reference more than anything, the answers above are the two main paid eCommerce plugins for Umbraco.
But the main free, eCommerce solution is Merchello: https://merchello.com