The store's website can be found here, and currently supports Windows Mobile, Symbian, Java ME, Flash, BlackBerry, Linux and Unix apps.
Categories include Games, but also Communications, Education, GPS, Multimedia, Utilities and a catch-all Various category. Here's a screenshot:
Developers can make their games free or charge for them, and even upload test versions to get early feedback.
At the time of writing, two games are available: Backgammon and SudokusWeb App. Consumers can rate apps from 1 to 5 on the site.
According to this interview with O2's James Parton, if Litmus is a success, it'll be launched in other countries via O2's parent company Telefonica.
It may also provide a new route onto O2's main portal for popular games.
"Those applications which our customers use and like the most will rise to the top of the charts," says Parton. "Then we can come and say 'those are the apps our customers like,' take them and launch them into our main customer base. Litmus will be like an "early radar" for us.
There are some quirks. Like the ability for users to pay in dollars or euros as well as sterling, despite the fact that only O2 UK customers can access the Litmus store.
However, Parton says O2 is aiming to approve applications within 24 hours of being submitted by developers, and that the operator will "do a lot of PR around the successful case studies who made it through the program".
Obviously, iPhone apps aren't included in Litmus, despite O2 being the device's exclusive operator in the UK. Intriguingly, Parton says O2 is "in talks with Apple to look at how we can work together in partnership".
