Screenshots

Games

OpenRA
A modern cross-platform reimplementation of the Command & Conquer: Red Alert engine using ICSharpCode.SharpZipLib, OpenTK, SDL2#, SharpFont and Mono.NAT.
Second Life
Second Life uses Mono as an engine that accelerates the execution of user provided scripts by translating LSL scripts into ECMA CIL bytecodes and in turn letting Mono turn that into x86 machine code.

Gtk# Applications

Banshee
Banshee is a music organizational and playback tool.
F-Spot
F-Spot is a photo management application for the GNOME desktop.
GNOME Do
GNOME Do is a crazy-delicious launcher tool that makes performing common tasks on your computer simple and efficient.
MonoDevelop
MonoDevelop is a free GNOME IDE primarily designed for C# and other .NET languages.
MonoUML
MonoUML is a CASE tool designed for allowing all UNIX/Linux developers faster computer systems design using a friendly GUI application.

OpenVPN-Admin
OpenVPN-Admin is a GUI for OpenVPN that runs under Linux and Windows.
Pinta
Pinta is a paint program designed for simplicity and ease of use. It is modeled after Paint.Net.
Tomboy
Tomboy is a desktop note-taking application for Linux and Unix. Simple and easy to use, but with potential to help you organize the ideas and information you deal with every day.
Smuxi
Smuxi is a flexible open-source cross-platform IRC client for the GNOME desktop.

Winforms Applications

Open Source Applications

NClass
NClass is a UML modeling application written in C#.

Paint.NET
Paint.Net hasn't been fully ported to Mono yet (due to p/invokes), but we can still see it using the new 2.0 controls.
SharpChess
SharpChess is a free computer application that enables you to play chess against the computer.

Reversi
Reversi is a popular game that is fun, requires just a few basic elements and has simple rules.
MonoCalendar
MonoCalendar is an application designed for easy management of calendars.
NUnit
NUnit is a unit testing framework that is open source written in C#.
MoMA
MoMA is a tool that helps you identify issues you may have when porting your .Net application to Mono.

Commercial Applications

Plastic SCM
Codice Plastic SCM - Commercial, Cross-Platform Source Control Management system for Windows (not pictured), Linux, and Mac OS X.
Likewise
Likewise Administrative Console (LAC) - ´╗┐commercial, cross-platform ´╗┐graphical administration console for Windows (not pictured), Linux, and Mac OS X.
Nu Tech Software Solutions
A virtual keypad and graphical status for Ademco Honeywell alarms.

Client-server socket support, write custom modules with supplied API and examples to perform your own tasks upon alarm events, customize your own buttons and images and much more.

ASP.NET

mojoPortal
mojoPortal is an open source content management system and web site framework portal for .NET on Windows or Mono on Linux and Mac OS X.

BlogEngine.NET
An innovative open source blogging platform developed with ASP.NET 2.0.
SplendidCRM
SplendidCRM was built on the Microsoft technology stack using Visual Studio 2005 Professional, but also runs on Mono.

Graffiti CMS
Graffiti CMS allows you to quickly publish and maintain dynamic content Web sites with little or no knowledge of Web programming.

Other Mono Applications

Unity
Game Development Tool implemented using Mono.
Maemo
Mono can be run on embedded devices like the Nokia 770. A video of the Nokia 770 running Gtk# is available here.
Eclipse
Eclipse is an IDE written in Java. It can run on Mono using IKVM, the Java VM for .Net.
Eclipse on IKVM