Opera, mostly known for their web browsers, has partnered with Appia and launched Opera Mobile Store. The app store includes free and paid apps for all major platforms except IOS, which will be added in the near future. The store can be accessed through any mobile browser, but is best accessed through one of Opera's browsers. When you navigate to the site it automatically detects your OS and brings up the appropriate apps.
As an Android user I personally don't see any reason for me to use Opera Mobile Store over the Android Market. It just doesn't make much sense to pull up a browser and navigate to the site when the market is just one click away. With the launch of the Amazon app store coming soon, I just don't know how the Opera Mobile Store will fair in the app store wars.