Spaced repetition is a method for memorizing stuff by reviewing it at programmed intervals. It works best for small chunks of data, like words of a foreign language. Memo is probably the simplest spaced repetition program in existence that is actually usable. At the time of writing, I have been using it daily for about nine months (while, during the first four or five months, tweaking the spacing algorithm from time to time), mainly to help me memorize Spanish and French vocabulary.
So why should you use such a tool ? There are several reasons why learning new vocabulary is hard. One is that you should practice every day, so you should be able to do it under any circumstances, and making use of any spare five minutes; or you just won't. Another is that if you do persevere, take your notes everywhere you go, and practice every day, you'll likely get demotivated reviewing older lists, wading through all the words you already know, looking for the ones that you really should practice; so you decide that after some point it's not worth the trouble any more, and you never learn the hardest 10 percent of any list.
So it's hard not because it's intrinsically hard, but mainly because the traditional approach is not efficient, and simply no fun. Accumulating lists of words and reviewing them from time to time is a demotivating hassle. To make learning vocabulary fun, or at least not mind-numbing, and at the same time to go for the 100 percent instead of the 90, you need a personal assistant who knows which words you have trouble remembering, and when you last practiced which word; and who can organise a five-minute quiz on the spot, asking you just the words that you either have not fully memorized yet; or learned long ago, and might now be on the verge of forgetting.
There are applications which organize such a process for you, but they tend to be big and complex, and they seem to be tailored to be used with huge lists of prepared items and go for a maximal result/effort ratio on them, instead of going for the 100 percent result on your own modestly-sized self-made lists. The tool that I wrote on the other hand goes for the 100 percent; is meant to be used for only a few minutes every day, is small and simple, and does exactly one thing, unobtrusively, in the spirit of UNIX tools. It must be said though that my tool is much more hacker-friendly than user-friendly.
I run memo on my laptop; a PDA might be even better, although I like to have a full keyboard. Let's say that I just heard or read a new word, like the Spanish word "sastre" which is "kleermaker" in my own language. Then I open the file "spanish.memo" with any text editor, add two lines:
? kleermaker
! sastre
and continue doing what I was doing before. This takes less than 10 seconds and no mental effort. Later that day I will run my tool ("memo spanish.memo"), which will then quiz me on the words that it thinks I should review that day, and which will also schedule the new word for tomorrow's quiz.
The tool only starts to show its value after a while, when over a 100 words have been entered spread over some period (I rarely add more than 20 words in one single day or more than 50 in one single week), so that the spaced repetition can start shining; but right from the start you should run it every day, or at least every day that you open your laptop. In the beginning, when there are only a few words in the database (i.e. the flat text file), running memo will often have no effect at all: that means that there is nothing to do today.
Note that new days start at 4 AM. It's sleep cycles that count, not time.
I had hit a bit of a plateau learning Spanish before I made this tool, and now I notice that I'm learning again, mainly by just watching the news on Spanish television once or twice a week, and making memo items out of new words I hear and often understand from context (news stories are quite redundant and news readers speak very clearly). It takes just a few minutes a day to keep up with my slowly but steadily growing database, currently containing 968 items, and it delivers a daily dose of instant sense of accomplishment.