Oracle has a very large and active user community. In addition to Oracle's support organisation (Metalink), there are newsgroups, mailing lists, blogs, podcasts and a wealth of technical resources available to developers, DBA's and consultants.
Siebel has a far smaller user community so I thought I would try to track down some technical resources that might be useful for Siebel techies.
SupportWeb is Oracle's official support site for logging Service Requests. SupportWeb also contains FAQ's, TechNotes and alerts for all Siebel products.
Metalink also has an increasing number of SR's related to issues with Siebel and the Oracle Cost Based Optimizer (CBO).
ITtoolbox is probably the most comprehensive, unoffical resource with a knowledge base, mailing lists and Wikis. Be aware that some of the material is pretty dated here.
Although there are no forums dedicated to the Siebel product, the technical forums at Oracle Technology Network (OTN) contain a small number of postings related to Siebel.
I also hunted high and low for blogs about Siebel...
If you know of any additional useful Siebel resources, please leave a note below.