I think this is a good idea, and something I have been investigating for here.

Intralaunch works well and is easy to use, but is pretty expensive, or as someone said here LaunchinIE works in a less sophisticated way but is free.

However, I have given up on the idea so far, for the reasons listed above. Its not likely to work in the longterm due to increasing security, and really limits you to what browser users can access your site with.

Something we do use (for a different purpose) is 2x, which requires a client install, but comes with a web portal, allowing you to launch applications, it basically just downloads an xml file which runs the application launcher.