A long time ago I bought a suite of tools from Adobe. Mainly for Photoshop and After Effects, but they threw in Illustrator as well. I had had a really frustrating time years before with Illustrator, so I kind of "forgot" that I had it. A few years later I saw some Illustrator workby an artist that I admired that seemed to mesh well with where my personal work was going at the time -all flat and stylized. So I busted out Illustrator to see what I could do with it and to my surprise mastered it very quickly. I did a lot of personal work with it and continue to hold it in my arsenal today.
One place I really got a work out was in doing forum avatars for my guild's website. It started by making images for me and my wife, but then everyone wanted one. So I ended up doing toon versions of somewhere over 60 guildies. We even opened a Cafe Press shop so we could buy mugs and tshirts. It was a lot of fun. I did a few custom pieces as well ranging from illustrating a crafting guide to a few special tshirts.