While I was in China the forum engineers pointed us at some of the more popular forums.  These forums had an interesting Virtual economy.  Rather than simply rewarding points for participation users could also exchange their reputation between one another.  One example of this is http://iask.com/.  Our friends in Shanghai translated their rules for us for consideration on Microsoft communities...

Earn Points By...

• Register and get +20
• Activate email box successfully +20
• Any time you sign in +5 each day
• Submit question or reply +1 for each post
• Marking an answer +1 each time
• Reply marked as an answer +10+the question's offered reward point
• vote or submit comment successfully +1 points each time (10 per day at most)
• Your question becomes popular +10

Spend or Lose Points By...

• set "offered reward" points bounty when asking questions -offered reward points
• don't set question status when it expires -20
• cancel the question when it has answer already -20
• exchange small gift -gift's points

Other Notes...

• Vote: when there are multiple answers, you may set a vote to let other participants to select the best answer for you
• The question expire time is 15 days by default
• If there is no answers, you can increase your "offered reward" points and get another 15 days expire time.
• When a question expires, you can still handle it such as set answer, and etc.

What other non-paid membership communities have you seen successfully implement this sort of system?  Do you like this sort of system?