By combining tagging, RSS,and AJAX, Flashcard Exchange makes it possible to create, study, and share flashcards in...well, in a flash.
Let's see how they work. I need to learn the color words in Portuguese (azul, vermehlo, etc). Fortunately, someone already made a set of cards on this topic. To practice with the cards, I choose "Study" - the icon with the lightbulb. A flashcard appears on the screen. Pressing "C", advances the cards. I mark my mistakes with "X". If I want to reinforce the words, I can practice more with a game of Memory. After signing in, you can also copy flashcards to the clipboard and save favorite sets for later studying.
Flashcard Exchange has a large archive of already made flashcards, including sets on some tough medical, legal, and philosophical themes (i.e. Kant's Groundwork of the Metaphysics of Morals). Creating new cards only takes a few steps. Updates for tags are broadcast via RSS.
With a one time fee of $16.50 USD, you can get the ability to print and download the cards. If you study a lot on your PDA, it might be worth the investment.