I remember using ChemDraw and the 3-D version almost 10 years ago to model nanotubes and nanopeapods (right after peapods were discovered, in fact). This free program does what they did and much more. It's a lot of fun to build little molecules and see how they form, especially when you turn on the auto-optimization that calculates how the molecule would actually position the component atoms.
It really does let anyone play around with building and interacting with molecular structures.
There are some weird interface issues. First, the keyboard commands to switch between tools are function keys, and don't seem to work on Macs, even if you toggle the function-key setting in System Prefs. Second, closing the last window quits the program. The fragment library doesn't appear to work right now, so it isn't possible to import the sample molecules that come with avogadro.
Nevertheless, if you're interested in molecular engineering, nanotech, or anything in that category, this is a free way to have access to a tool that lets you play around. It looks like it has much more advanced features as well, but I can't speak on those with any authority.