Review of Mnemosyne vs. Anki vs. SuperMemo http://nihongoperapera.com/mnemosyne-anki-review.html
Open Source Anki: http://en.wikipedia.org/wiki/Anki
Open Source Mnemosyne: http://en.wikipedia.org/wiki/Mnemosyne_(software)
Some science: http://en.wikipedia.org/wiki/Forgetting_curve
http://en.wikipedia.org/wiki/Spaced_repetition
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And my favorites for language learning: http://www.smart.fm/
http://projects.ict.usc.edu/itw/gel/EricssonDeliberatePracti...
I'd also say you need to do it intensely - you've got to be into and "feeling" what you're doing. If you're just sitting there waiting for your 3 hours of "practice time" to elapse so you can go chill you may as well just not do it.
The problem with favoring long sessions is that you can fall into the trap of only doing long sessions because of the perceived efficiency. Which means you'll need to 'make time' for it. Which means you'll put it off. The result is that most of the gains you make will be lost by the next time you find time for another extended session.
I found out about it from this article, which was posted on HN a while back:
http://www.wired.com/medtech/health/magazine/16-05/ff_woznia...
(I would be surprised if this problem wasn't solved in the up-to-date, PC versions.)
I still like the idea, though; I've been using Mnemosyne (http://www.mnemosyne-proj.org/) recently to practice my Spanish vocabulary to good effect.