I think it would be a good\nice idea for an almost retrospective bounty award. I have been helped out several times by people and if they were someone in the office I would buy them a beer to thank them. I think there should be a SO equivalent where if someone gives a good response you can 'Buy them a beer' i.e You give them 20 of your rep to say thank you. I know we vote and accept answers but it is not as 'personal' as it doesn't cost the OP anything.
If you want to be super nice, pay attention to their profile page as well as any comments or answers they leave. Get a feel for them as well as picking up any salient information as to their real world location/identity.
After some investigative work tracking down their details (shouldn't be too hard given some are wanting to display any number of ways to contact them) fly, drive or boat yourself out to their house/flat.
Knock on their door and invite them out for a beer, coffee or what have you.
Because if you want to show someone on the internet that you really care, you'll turn up unannounced at their door at 2am. Then you can give them a real world high five.
You could see if they've written a book, and buy a copy if they have ;)
You could just go into that user's profile and upvote some other thing that he has done, if you wanna be nice.
You have a point...
I've seen comments to the effect of "I wish I could upvote more than once" etc so this might fit the bill.
Perhaps a "throw my beer in your face" badge too when you can't downvote enough...
Related question - Badge suggestion: gold badge: beyond-call-of-duty
I think it would be great to have a way to nominate a user for being extra helpful, rather than just upvoting. I don't think you should be able to nominate a user more than once, but with enough of these helpful nominations a special badge or designation would be nice for these folks.