Some of my friends and I use Venmo to transfer money to each other. It's a startup company, so it hasn't caught on like Paypal. But their aim is to provide a free alternative to the fee-charging money-hungry payment transfer institutions.
For member to member purchases through forums, I've almost always use Paypal cause everyone seems to have it, and occasionally Amazon Payments. There's also Google Wallet, but I've never tried before. I'd like to see people's thoughts were about the other payment methods, and whether or not they'd be good for informal transactions like on geekhack.