Want to do more with Twitter than just search and send messages? Twitpay lets you send payments via Paypal to other people using the popular social networking service.
On one level, Twitpay relies on trust: you send a message in the form '@username twitpay amount', effectively making a promise to pay that amount of money via PayPal. To actually use your account to make real payments, you need to do several things: start following Twitpay's account, "claim" your Twitter username, and then associate your PayPal account with it. (The "following" stage is necessary so that Twitpay can send you a direct message with your PIN for the service.)
The big downside? The service isn't free: for any payment of $1 or more, you'll be charged 10 (US) cents by Twitpay. As such, it might be easier (and cheaper) to send someone a direct message and asking for their PayPal account email to organise a direct payment. However, if Twitter is your main communication medium, it's something to consider.