Twollars allows you to send virtual money with simple tweets. Every person on Twitter is credited with 50 twollars to pass around. You can use them for good deeds, for a thank you or just because of the effort someone puts into their tweets. Aparently these can be sent to non-profits and even into real cash. Sending them would look like the following:
Give 2 Twollars @macwind because he gave me great advice
There is no registration on their site and you can see your balance by simply entering your twitter username into their homepage. It will show your balance. Hopefully an idea like this, or this one, catches on and we start trading some form of currency around Twitter making it an eco-system.