Don't think the ridge at the base of the 737-800 tail has been mentioned yet - helps distinguish 737 and 757 (if you can't see the wheels or are at a distance, and especially since more and more of the 757s have been fitted with winglets).
Also, to address a query above, the A300 does have 2 rows (pairs) of wheels on the main gear, but I'd still go by the winglets.
The regional jets are tougher - this is the best I could come up with back when I was trying to figure these out:
Canadair CRJ
- 2-engine regional jet with T-tail
- top of tail has a rounded bump toward the front
- evenly sloping nose
Embraer 135 / 140 / 145
- 2-engine regional jet with T-tail
- top of tail slopes downward
- sharper angle at the nose
Side-by-side, the CRJ is larger and wider, of course, but on their own, they can still be tough to identify.