I'll slow down and ask if they're OK or need any help if I see a cyclist (road/mountain/bmx/whatever - we're all cyclists ffs) faffing at the roadside when I'm cycling. In a car it's a bit awkward - I probably wouldn't stop unless the person looks really in trouble - a car slowing down and winding down the window often freaks people out. Last weekend I was in a car behind somebody on a bike with open pannier straps flapping scarily close to the back wheel - decided to let him know, so as I passed I shouted and gestured to him - after he'd realised I wasn't abusing him he was quite greatful, but it was a strange few seconds.
Some people deserve to get stuck though... I offered help to a bloke out on the moor some time ago - turned out he'd got a puncture but had no spare tube or working pump. It was a massive cut in the tube, obviously from running the tyre way too soft for the rocky track. I didn't have a tube the right size on me, so I tied a knot in his old tube to cut off the split bit. Bit lumpy, but it works as a get you home bodge - but as he was in the middle of a day ride I offered to let him have a tube if he followed me home a couple of miles away (the way he was going anyway). The muppet refused to put a decent amount of air in the tyre ("that's how I always have it") and got about a mile before he sliced the tube again. In the end his mate followed me home to get the tube and went back to join him - wonder if that one lasted the rest of the ride?
Grr.
But yes, I do try to help other cyclists, and hope the same would be true if the situation was reversed.
Rob