I found those a while ago, excuse me if they have been posted before.
http://graphics.uni-konstanz.de/plantslib/
these are modeled with Xfrog, the big tree i DLed has 0.8 mil polies.
Yes, these have been mentioned before. However, there have been some issues with the alpha textures which, coupled with the fact that the plants are not very detailed, have kept most of us from using them.