drmike
100% Tier-1 Gogent
Would be an uber good resource for a howto of some of these high availability self rolled solutions. 
On the cheap and commonly for static content we use Rage4 DNS with multiple front ends distributed geographically. They have monitoring hooks (API) for detecting offline nodes (unsure if their features at this time go beyond there to catch intermittent issues).
When dealing with databases and users that are creating data, the puzzle is far more complicated. Master-master MySQL replication seems to be the next size up solution coupled with this and rsync to replicate files both ways.
On the cheap and commonly for static content we use Rage4 DNS with multiple front ends distributed geographically. They have monitoring hooks (API) for detecting offline nodes (unsure if their features at this time go beyond there to catch intermittent issues).
When dealing with databases and users that are creating data, the puzzle is far more complicated. Master-master MySQL replication seems to be the next size up solution coupled with this and rsync to replicate files both ways.