I choose to create the trees for our games project, and while there was an option to try and find a tree generator and tweak the stuff, I took this opportunity to learn zbrush a bit better.
I got some really nice shapes from using zspheres, they were based off bonsai trees as I felt this style of tree but much bigger would quite fit the sort of fantasy/surreal feel which the game is going for.
I then imported them into 3dsmax, and optimized them a bit. My plan is to take them into the engine and just move each one around a bit, scale and that sort of thing so they don't all look too similar. I might also come back and try and place one of these trees cover a rock or something.
Other set pieces I have left to create - rocks, plants, bushes, hedges, cliff like pieces and shaping the terrian!