GUI Toolkits - More than you can shake a stick at

It seems to me there are loads of GUI toolkits out there but virtually none reach maturity, unless pressed by a commercial interest/funding or somehow they manage to reach critical mass with respect to their community activity.

Successes certainly have been:

• Qt - thanks mainly to being used by KDE to bring it right into the developer consciousness. It also helps the design is really quite good.
  
• wxWidgets - now the community is getting seriously large and active. The comprehensive nature of this library and its maturity helps to draw people in.
  
• Whatever Microsoft technology they are pushing today - WinForms? Avalon? Sheer commercial brute force will get people dumping whatever they were told to use last time in droves and get them to implement it in Microsoft's Golden Child Of The Moment.
• GTK+ is being used in Gnome. It is certainly well-developed and well-used.
  

In fact it is quite scary how few commercial GUI toolkits there are out there. You are normally given a low-level set of APIs from the desktop environment and if you are lucky you get a higher-level API (which will definitely not be cross-platform designed).

Anyway, to the point of the post... You can check out piles and piles of GUI toolkits here.