Very few people realize how important Mesa has been in the success of linux. During the late 1990's it enabled dozens of people to migrate from SGI and other workstations over to PC hardware running linux.
A very clean API and high level widgets for managing visualization tasks in a cross platform and highly configurable way. KWWidgets is particularly well suited for people using VTK because it inherits the programming style and build system meaning your code is consistent. Once you have learned VTK, kwwidgets is a natural extension.
I have used tkcon for a loooong time (maybe 10 years?) and find it absolutely indispensable. Not only do I add a console to all the programs I write, but I even use tkcon as my programmable calculator of choice!