Online messaging for websites?

I'm creating a messaging system for a website with multiple users, and linking it to a downloadable messenger style application. I'm need suggestions on what platform to use for the development c++, VB, etc, and any feedback on how current messenger systems could be improved!!

If you intend for the installable application to be very widely used, you should probably write it in c++. However, it is MUCH easier to write it in a managed language/framework like c# or VB.NET. It will require that the users have the .NET framework installed, but that isn't as big a deal as it used to be (pretty much anyone with XP or Vista will have it pre installed).

If you write it in c# .NET, you will also find it fairly easy to write a Mac version with Cocoa... many of the development concepts are similar.

As far as improving on existing messaging systems... lets be honest. AOL, Yahoo! and Microsoft spend millions of dollars a year paying some of the most talented developers available to work on their messaging platforms. It's not impossible for a lone person to have some amazing idea that will blow them all away - it does happen - but it's extremely unlikely. And if you do have your killer idea, you will have to be a killer programmer to be able to implement it. These platforms are extremely complex, built on custom communications protocols, and in general require many developers to maintain and enhance.