I've downloaded and used many of the programs mentioned in various posts. Today, the main 3 programs I use for plants/objects are:
Grass: Bantam 3D grass
Why: multiple blade shapes per tile, create grass clumps, multiple groups per grass object for additional variation, easy to use
Trees: Arbaro
Why: easiest tree generator to use (IMO, graphical diagrams help explain settings), easy to setup different versions of the same tree/object. , samples provide good starting points for different tree types. Just remember to give java lots of RAM
OBJ conversion: <edit.. minor blockhead moment>PoseRay</edit>
Why: easy to fix baseline problems, converts materials to groups if necessary, most stable I've used when imported into TG2 (compared to Blender & Wings)
I haven't done much in the way of buildings, but I have had a play with Google Sketchup and it's very easy to use. The registered version supports file export into usable formats and there are a lot of models that other people have shared of real buildings.
