At $2 this app is a heck of a lot cheaper than buying any the materials necessary for your own dark room, and I can guarantee it's less messy too. Though not quite as fun, I know.
Wired got hands-on with the Swankolab iPhone app, and found it a whole heap of fun, changing a photo from the iPhone library with various chemicals which perform different effects. We're talking vignetting, saturation, the usual retro lomo-esque effects that are very popular nowadays, as seen in the Hipstamatic app—which has the same developers as Swankolad. The app "pours" the chemicals into the bath, and once you dim the lights the photo starts to develop. Putting that in-app purchasing to work, you can also buy other chemicals for even zanier effects, at a further $2. [iTunes via Wired]