Things like manuals, wiring diagrams, hard to find/obsolete parts, how to diagnostic info, etc. Be nice if the members had go to forum that had this type of info. Could even close/lock all the started threads so no one can post to it.
Examples:
How to check starter solonoid with ohm meter?
How to adjust valves on a Briggs vtwin?
Sears to Tecumseh engine model cross reference
Honda GCV160 service manual link, free download
Stihl 011 parts manual, free download