Tipping on good answers has been dealt before, so I'm not suggesting that here. And, of course, we are able to pass on a few rep at any time to a deserving person. Nothing wrong with that option, except that the reward isn't tied to the helpful behavior in any meaningful way; It's clear that thankful people frequently want that association.
What I would like to suggest is a mechanism for directly tipping someone for being particularly helpful on a comment or in a chat room.
It would need to be:
- Visible to at least the tippee where it came from
- Cost the tipper some rep
- An unexpected reward after the fact
I think to satisfy condition 3 it would have to be limited to a relatively small amount, so people aren't inclined to shop questions around chat offering bounties in advance, for instance, would be useful.