I'm not sure what you mean by clustered servers, but I think
your referring to a cluster solution. In that case, Oracle has 2 solutions.
1. Oracle Fail Safe.
FailSafe uses cluster technology.
Ie Oracle software is installed on 2 nodes and the db resides on a shared disk. If the instance fails on Node A, then the instance on Node B takes over. failSafe does not share the resources between nodes.

Parallel Server in essence in the same but it allows sharing the resources between multiple machines for the same db. I believe 9i will also include Parallel Server Failover (ie with failsafe)...