Applorer is an app discovery tool for the Android market that has almost 100,000 installs, and with a recent update could help developers get some free marketing for their apps.
The new update allows users to 'recommend' apps they like in their social outlets, and discover apps that their friends are currently playing and recommending. The opportunity exist that one of your apps could be recommended in a social circle that instantly lets you get exposure to an audience you might not have otherwise.
It's an interesting marketing concept that attempts to bridge the gap between using mobile social outlets, and what apps are on the mobile you are using.