Here are some thoughts:
- I did this in the past using Red Hat Cluster. It worked fine.
- We have this with some of our databases using HP Service Guard.
- Most of our new databases are going on VMware, so this provides VM failover, which gives similar HA to active/passive.
- Oracle provide something called RAC One Node, which is a regular RAC installation, but the instance only ever runs on a single node unless a failure occurs.
- There are standby solutions, that give you similar results, such as Oracle Data Guard or DBvisit Standby.
Each of these solutions have different licensing implications, so you have to understand what the technology gives you for the money you have to pay.