CounterWallet runs in your web browser.
We have had consistently good results using Chrome and Firefox.
Testing has been less thorough on Internet Explorer and Safari and so we do not recommend using these browsers for this application.
With Counterwallet, your passphrase is literally your wallet, and all of your addresses and keys are generated on-the-fly when you log in.
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.
Official Counterparty Counterwallet Server
CoinDaddy Counterwallet Server