BitCoin is a free and open currency that will exist safely and decentralized online. Read about it.
This method is reported to work on 10.04 (Lucid), and I can confirm that it works on 10.10 (Maverick). I don't yet know about 11.04 (Natty).
Open a terminal
sudo apt-add-repository ppa:stretch/bitcoin
sudo apt-get update
sudo apt-get install bitcoin
To run BitCoin, navigate to Applications -> Internet -> BitCoin.
Also, remember there's Flattr, too!