Popling lets you "learn without studying" (or annoy yourself)

Distractions... can't live with them, can't live without them. At least that's how it goes for me. Popling tries to take that tendency and make it into something productive. It's a flash-card type application (stand-alone or Firefox add-on), which pops up every few minutes while you're busy doing something else and queries you on one of a set of questions included in your "Popling." A Popling is like a stack of cards on a specific topic. The site itself does not seem to feature that many Poplings, but you can make your own.

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?