Sketchup users do alright with the tools that come with the program, but I think any experienced modeler will agree that plug-ins can be a huge help in terms of capabilities and work flow. There are approximately a gajillion ruby script plug-ins now available (ok, last time I checked it was 957), and, well, some are better than others. One of the leading authors of plug-ins is Didier Bur, a virtual saint in the SketchUp world, and I recently had occasion to use his Compo Spray plug-in.
I was impressed with its ease of use and versatility. It comes in very handy when you need to place a lot of components and want to achieve a plausible degree of variety and/or density. Here’s how it works: First, you define the set of components you want to use (say, a few different kinds of trees) and in what proportion (maybe you like maple trees twice as much as alders). Then, you choose the spray “shape”–14 different ways that components can be arrayed–and pressure (0-100%). For my purposes, the “selected faces” option at 80% pressure allowed me to quickly forest a section of terrain. You can also set a number of other, pretty self-explanatory options such as whether or not to keep components vertical (useful for trees!) and scaling (nice to achieve realistic variety). Then, click the green “spray” button and sit back as the computer populates the model. Voila! (The 21-page pdf that accompanies the script is worth reading–it lays everything out very clearly, and there actually are that many features to discuss.)