what about Open Transactions? It doesn't have its own cryptocurrency, but allows you trade in all kinds of currencies, just like Ripple. And unlike Ripple it has already been open-sourced.
The problem with Open Transactions is that it doesn't satisfy immediate consumer demands. It's a great system (although a little bit complicated) but it seems to be a solution waiting for a problem.
Bitcoin solved a problem that has plagued mankind since the inception of money.
Ripple continues in that tradition by solving new problems that Bitcoin cannot solve (confirmation time, working with IOUs, distributed order books, etc...)
For a system to become popular like Bitcoin, it has to serve current demand. Both Bitcoin and Ripple do this, but Open Transactions does not (although it hopes to someday).