At my work we found this great but secret project called Sequoia. It is an open source project that can increase the performance and uptime of your database system. The remarkable thing with Sequoia is that you can use the project without changing your application or database schema and you can use it with any database server you like. It almost sounds too good.
The software implements a RAID like solution on the database connector level. In other words. The thing you change in your existing system is the database connector (JDBC, ODBC, or what ever you use). The Sequoia connector will RAIDb you databases accross several servers a bit like RAID controllers do it on disks. If one of the nodes in a RAIDb-1 controller fails the Sequoia connector has a transaction log that you can replay on the node when you get it online again.
In theory Sequoia and enough hardware will let you scale you database to any level of redundancy and performance.
Disclaimer: I have not tried the Sequoia software yet. All the facts above is from their own documentation. I will post my test results later.