Viktor Pilpenok wrote in a message to David Drummond:

 VP> The basic formats for 3.5" floppies are: 360k (1 Side, 80 Tracks, 9
 VP> Sectors/Track), 720k (2s 80t 9s/t), 1.44m (2s 80t 18s/t) and 2.88
 VP> (2s 80t 36s/t) - all of the extended formats are based on those 4
 VP> (actually the last 3 - 360k didn't have any extended formats).

 VP> Mostly it went by increasing sectors/track like this:
 VP> For 720k  - 10 (800k), 13 (1.04m) and 14 (1.12m).
 VP> For 1.44m - 20 (1.6m), 22 (1.76m), 23 (1.84m) and 24 (1.92m) For
 VP> 2.88m - 39 (3.12m), 40 (3.20m), 44 (3.52m) and 48 (3.84m) 

 VP> There were also attempts to increase capacity by increasing the
 VP> number of tracks, but those never became too popular because of
 VP> problems with compatebility and reliability:
 VP> For  720k 10s/t - 82 (820k) and 83 (830k) tracks
 VP> For  1.44 21s/t - 82 (1.72) and 83 (1.74) tracks

I thought I remembered a "1.68M" format from someplace,  too, 
for whatever that's worth...

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