Popling lets you "learn without studying" (or annoy yourself)
I'm not sure how active the site is, actually. The first couple of times that I clicked the Learn link, it gave me a weird error message. The site also has a bit of a run-down feel to it -- like it's not carefully maintained. However, I don't know of any other flash-card applications that interrupt you dynamically like that (unless someone posts something better in the comments), so I certainly intend to try it out myself. Note that I don't mean a dedicated application which works with "study sessions" (there are plenty of those around), but rather,something that actually integrates with my normal work and just pops a message up every now and then. What are the other contenders in this field?