Ars Technica has a tutorial on one of my favorite gardening methods because of the science involved. That and because it's really neat. Learn the technique and you'll have many more plants than you thought you could afford. In a nutshell, you cut pieces off an existing plant and help it grow roots until it is a complete plant on its own. The easiest common plant to start with is tomatoes, which is what Jacqui Cheng uses to demonstrate. I do this with different kinds of flowers, mostly begonias this year. Link