Acid: expensive, kinda hard to come by, awful hangover. But the pretty colours -- I know. Consider Photojojo's Double Exposure Digi Cam instead: it'll merge two images (or videos!) into one warped, dreamlike scene. And it's very small.
It may not be super cheap for a novelty camera -- $US130 -- but the effect looks like a lot of fun, and as mentioned, it's tiny, so tucking it away in a bag is a given. The 3.2MP sensor might also sound disappointing, but the allure of the distorted double exposure is its gritty (lack of) fidelity; think of it as being part of the ironic photo filter family. And hey, you can always use it on acid. [Photojojo]