SnappyTree is a WebGL-based procedural tree generation app by Paul Brunt, of GLGE fame.
It offers a bunch of neat presets, along with 20+ parameters that affect the trees ‘growth’ and texture. Your resulting tree can be output to JSON (for use with proctree.js), or the Collada / Wavefront formats, for native use.
If you’re interested in learning more about procedural object generation, this Wikipedia article on Lindenmayer systems may be of interest 