Not an invention , but a discovery...
For me it was realising in 1996/7 that you could get a listing on a link list that was only frequented by credit cards holders.
It was the best feeling EVER seeing it work.
Building thousands of AVS sites a month watching them get like 100 hits a month each , solely from the various AVS sites link lists , and getting a upsale per site per month.
It was awesome.Went from scraping by on the net , to a millions in a matter of months.
No feeling like it....
Well until we discovered cloaking, dynamic , etc in 98ish for the Search engines.
The AVS trick I discovered , the SE shit ...ummm...we ...ummm.. "acquired" the basics