This has come up several times, so I figure maybe it's worth posting in its own thread, and I think I'll stick it as well, so that folks don't have to remember exactly where to find it.
I'm pasting this from another thread, so if it looks eerily familiar to anyone, that's why.
Once again, thanks to Alexander Turcic for explaining the whole thing to me in the first place (now there's a guy who deserves karma!).
MobileRead's Karma system
Each person seems to start out with 1 point of influence (after they've posted at least once), then it follows a formula. Here's the formula that I worked out from what Alexander told me, it seems to hold true from what I've seen:
(# of days registered/30) + (# of posts/40) + (Giver's present
karma/200)
and the result will be rounded down to the nearest integer
Remember, these are all based on the
karma giver's current stats.
So, for example if a person who had been registered 300 days, had posted 2400 items, and had a present
karma of 400 were to give someone else
karma:
(300/30) + (2400/40) + (400/200)
Or:
10 + 60 + 2 = 72
So If this theoretical person were to tap someone who had, say 28
karma points already, the tapee's
karma would increase to 100 points. And yes, I picked nice, tidy numbers to make it easy on myself -- I'm used to the computer doing my arithmetic for me (and most of my spelling, too, for that matter).
The biggest single factor is how much you post, the more you post the faster your givable
karma goes up.
Conversely, the only way for your
own karma to rise is for someone
else to tap you.
There are also some limitations to how much tapping you can do. I believe the deal is that you can't re-tap a given person until you've tapped two others, and you can only tap 10 people in a given day.
So remember, if you appreciate something that someone else has done, nothing says "thank you" like a
karma boost.
Disclaimer: this is in no way an attempt
.....to get my own karma boosted -- I'd
.....much rather get karma for stuff
.....that's more worthwhile than this.
.....Or at least more effort! 
Addition: Yvanleterrible reports that the blocks in your geekiness indicator
(which relate to number of posts in some not presently fully understood fashion) turn blue when you reach 400 posts.
One other note: you can see the entries of who gave you
karma for which post and what their comment was from the bottom of your user control panel (toward the upper left corner of the screen, for those who might not have noticed it yet).
edit : DixieGal has contributed this helpful visual aid to show where to find out who gave you karma. thanks DixieGal !
