Flap 2 - aka "the gentleman's flap"
I find it very useful. Either if given speed 180 at high weights with the '1' speed around 190ish, or as another method of losing excess height. Rather than speedbrake clean/250 knots, using F2 220/230 knots is a very effective way of getting back on profile (~1500fpm) and smoother than using the speedbrake.
I use Flap 10 almost daily on the NG, and Flap 2 at least a few times a week. Flap 30 is fairly often required as an intermediate setting if you've got a bit of a tailwind trying to make the gate for F40 too.
Now if anyone can tell me how to consistently grease Flap 40 landings...