The first off-sim products I ever saw (not that I searched thoroughly) were those offered by Tobias Novi. It was a series of coastal rock formations that extended way beyond the sim limits using a very simple trick that now is commonly used. By that time (I can’t even remember when was it, maybe 2-3 years ago) Mr. Novi also had some buildings, tree houses and some additional house plants. Since then, he has expanded his product line, and include among them a collection of low prim trees that are both likable and useful.
There are dozens of trees to choose from in Tobias Nobi’s inworld store, the most recent being of less than 5 prims each. My favorite ones are the Conifer A, B and C, not because I have any particular fixation with this kind of trees, but because they are available in four different sizes –individual trees, 10, 15, 20, and 25m tall– and in groves of eight (20, 25, 32 and 40m tall). Consider, for example, that you could use those 2-prim groves to fill a forest. Now, do you see why they are so good, besides being so nice looking?
These trees aren’t available in the SL Marketplace as of now, so you’ll have to TP inworld to get them [slurl]. While you’re there, take a look at the recently introduced birch set, also available in three sizes, at 4-prim each. (This time, the pictures were taken at the Mycenae sim).