Here are two ideas I'm still testing. Probably pretty obvious to you guys, but I'll post 'em anyway...
So, loops are easier to use, but they take a lot of floor space, which is a pain if your fort is cramped, or if you're moving stuff from distant points (ore mining) to a central hub. Then, keeping a route on one line is better. The simplest way to do this is to have a dwarf kick the cart, but then you have to wait for some lazy dwarf to actually walk over there and do it. Thus, the auto-returner. They take a chunk a space at the end-of-the-line, and machinery, but will--ideally--save space and time.
The EXCLAMATION POINT RETURNER:
Example A: ☼
O╞═ ≡ ═══■══^════════╢╢╢╢O
[s]Example B: bugged
In Ex. A, the pressure plate is a little less than 100 ticks before the rollers; it turns them OFF, then flicks them back ON after the cart reaches the end. This allows the cart to get all the way to the wall, and get powered by ALL the rollers instead of just the first one. (Although, you have to make sure the cart doesn't hit the wall and bounce out past the rollers before they even trigger.) In Ex. B, the end roller is always on, and the other roller is connected to the pressure plate/gear assembly; it turns them ON then resets them to OFF after the cart is sent away. B is easier to time than A, but takes more gears,parts,and power.
THE QUESTION MARK RETURNER:
Example C:
O╞═ ≡ ═══■══════╗ . ═╢╢╢╢╢╢╢╗
╚═══════════╝
This returner doesn't need pressure plates. If there isn't enough power in the rollers, the cart won't shoot the gap, and it will just fall in the hole. Probably suitable for a really long or windy track, or if you just love things going at maximum speed.