^^^
The first ones we got were from the BTI recording session (just stems, not multi-track stems), there's also full recording sessions for Gimme More, 3 and Toxic floating around if I'm not wrong.
Anyway, except for BTI all aother stems came from the Sony Music server when they were using the songs to make the Femme Fatale remixes last year. Some people got a way to grab all of that stuff for free and very easily, although Sony noticed that and now it has changed their system, so no more leaks for us
There are multi-track stems for:
Practically all songs she did on the FFT, plus Scary, Don't Keep Me Waiting, Burning Up (unfinished/unproduced), Criminal (I think they sent the Radio Mix, dunno why just the Album Version leaked), and He About To Lose Me.