Houdini is actually descended from the PRISMS package developed by Omnibus Computer Graphics (one of the very first CGI studios (and actually still in business in Japan!)). SideFX (started by two of the original programmers) bought the rights to PRISMS when Omnibus (North America) went under. They opened for business in 1987 and have been going strong since!
They have a number of "firsts" to their credit -- for instance, Houdini was the first major 3D package on Linux. Debuted in 1999, although I was beta-testing it before that.
For procedural 2D and 3D graphics, there is absolutely nothing like it. However, it's hard for most users to understand, so its popularity has never been great. Anyone doing serious FX work knows all about it, however.
(My old boss used to be CTO of Omnibus, in charge of PRISMS development.)
It has no (built-in) UI. Its file format is a domain-specific scripting language. :-D
Here is their demo reel: http://www.youtube.com/watch?v=leJks6ay4tg
Apparently Maya, Softimage and 3DS Max are not the only players in this space anymore.