b2n2 - a couple more to add - commercial (assuming it isnt initial licence), and the private checkride or any writtens. And before any nit pickers start up, that is correct, and I have declared these on entry under esta on various (4 in total) occasions and been let in without any further questions. One could argue that signing up for a commercial course as opposed to adhoc commercial training might need an m1 but why do it that way anyway?
Also, class ratings as opposed to type ratings are often exempt if one has an initial private certificate already.
For what its worth, when I came through dallas this week, I asked the immigration officer what visa would be required for a two or three day type rating or multi course, the answer was B1, and I then said I presume that also means esta is acceptable if one qualifies for that and he said yes.