There are quite a few Linux apps that run on Mono: Banshee, Tomboy, F-Spot, Beagle, Gnome Do. Moonlight (built on Mono) provides the Linux version of MS's Silverlight plugin.

Second Life uses Mono for scripting. Quite a few games (including iPhone and Wii games) are written using Unity, which runs on Mono. Mindtouch Deki is a web wiki/CMS that runs on Mono.

Not overwhelming, but not far from being a failure.