At $2.49 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 $2.49. [iTunes via Wired]