Daniel - I often use xy-pic for making diagrams, but these days TikZ is more popular among mathematicians, mainly because it's more flexible and can do more. It's attracted a large community of people who put [examples](http://www.texample.net/tikz/examples/) and [packages](http://www.texample.net/tikz/resources/) online. So, I've been getting my grad students to learn TikZ, and then learning it myself.