There are, of course, EAST and WEST, the two front end interfaces on the Office's patent database. It includes US patents and pre-grant publications (PG-PUBs), as well as patent documents from the EPO, JPO and Derwent, and IBM Technical Disclosures.
There are also a wide variety of non-patent literature (NPL) databases which are available to examiners. Most of them are somewhat specialized and are useful primarily to examiners who handle applications from a specific art. Some of them are paid databases, including DIALOG (although the use of that specific one is pretty much limited to the searchers in the STIC).
The thing is, the wide variety of searching resources places examiners in the awkward position of possessing an embarrassment of riches. There are so many places in which they can search, they simply don't have the time to use them all.
It has been my experience that for the average application, an typical examiner spends the vast majority of their time searching exclusively in the US Patent/PG-PUBs database. NPL is often under-utilized, which is too bad, because there are always those cases where the best prior art is NPL.
Why does this happen? Well, since examiners are given only so much time to search, they can only afford to spend their time searching the resources that they think are most likely to contain relevant art. For new examiners, they might not have a very good idea of what types of art can be found in each of the databases available to them. Since the US patent database contains a large body of prior art, it makes a certain amount of sense that examiners would concentrate their searching time there.
As examiners gain more experience, though, they get a better idea of what databases are useful for finding specific types of prior art, and so they are more likely to devote more of their time to search a wider variety of resources.
STIC helps this process along by providing quick (15-20 minute) tutorial classes for examiners on a regular basis, each presentation covering a specific NPL database available.