Schildbach BTC Wallet
Counterwallet is a Counterparty wallet that allows you to store BTC, XCP, and any Counterparty tokens like Bitcrystals and FoldingCoin. Counterparty does not require you to register an account. All encryption is handled client-side, meaning that your passphrase nor any of your private information ever leaves your browser.
There are no wallet files to backup or secure, and using your passphrase you can access your wallet from any trusted machine with a web browser.
Counterwallet is entirely open-source.