The big daddy in tree generators probably has to be Speedtree by Powered Games. For big budgets, it's not a bad choice in that it goes for around $10K a license, but it's a little much for smaller indies. It does do the job, but issues with it, it can be a bit difficult to put into your engine, the backend is a bit clunky and at the end of the day, the trees are pretty much billboard based so relies on the quality of the alpha channel work on the textures and it can put out a lot of polys.
Easy Tree Generator - This is a bit of a no thrills option, but at $20 it really isn't bad money for value, but with that, it is a little limited in the output, well you can and will spend a lot of time playing around with the slider settings before getting good looking trees that look varied. This also is pretty much a billboard style to the foliage, but at this price range not bad.
Ivy Generator - Okay, this isn't a tree gen, but it's cool all the same, creating ivy that grows around your mesh and looks awesome. There's also a similar max plugin.
Dryad - This is another university option that came about from Standford university and is worth having a look at.
A Siggragh 2007 Paper - "Approximate Image-Based Tree-Modeling using Particle Flows" This is just very cool and also worth a read.