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.
There are a ton of people using Unity for game development. Unity's quite awesome, and they get extra bonus points for supporting Boo: http://unity3d.com/unity/
Is anyone here actually using Mono for web or service development? Or even for desktop apps?