Apple themselves started moving the toolbar into the menubar (e.g. iTunes, Mac App Store).
IMHO the "left category bar" of Sparrow and Twitter for Mac are pretty useful, given that (a) screens are much more wide than high, and stacking everything in horizontal bars is a waste of space (b) they move "categories" from horizontal tabs to vertical, thus making the Y direction for 3-or-more finger gestures meaningful and intuitive.