It all started when Yahoo! dumped Google earlier this year in favor of its own algorithmic results. I hadn't paid much attention to Yahoo! until then (because if it was in Google, it was in Yahoo!). Sure I knew some things about it. Like getting a directory listing was difficult, to say the least. But when it comes to the finer points about Yahoo!, I was in the dark.
But when they did the switch, I started to pay more attention, obviously. Not just because they account for about 1/3 of most sites total referral traffic, although that's what initially got me looking at them, but because they also had a lot of features that I'd heard about but never used.
At the time they also released a toolbar (in beta) with a measurement called Yahoo! Web Rank, which is similar to Google's toolbar with PageRank.