GETTING INTO THE WAYBACK MACHINE: One great tool that I've used for a while is WaybackMachine.org. While some of my compatriots are familiar with this site, I was surprised that many others have never heard of it. For those of you seeking "other" prior art, this is a must-have for your invalidating arsenal.
What is it, you say? Well, the short version is that a group called the Internet Archive has started a huge effort over 10 years ago to archive digital content that has amassed on the Internet. Eventually, the goal is to create an "Internet library" that scholars, researchers and historians can access to view archived digital content and web pages (if you're interested in the long explanation, click the link above).
The database is particularly useful for quickly checking potential public use of patented technologies, through announcements and "news" reported on company website. Also, if you're dealing with standardized technologies (e.g., IPv4/IPv6), you can browse through publications released by standardization and trade groups during a specific time periods (e.g., January-May 1998) to see what was known in the art at the time. On occasion, you can get documents published by speakers that pertain to specific applications - if you get lucky (like I did last week), you can find a "killer reference" that can make you very, very happy.