I fell over it in the middle of the night one time!
I found AskMe first, because it was often linked in lifehacker. I lurked for six or eight months without signing up, and then was suddenly gripped with the need to answer a question about....something (and yes, I'm too lazy to go look), so I ponied up my $5.
Row, dammit, row!
I have many more than a single fondest memory, but first among equals is the (warning: enormous, browser-creaking thread) sansgras callout. It got longboated, and out of that endless rowing came some good friends. We still hang out, both online and in person. We've slept on each other's couches. We visit each other. It's really the thing that kicked me from being slightly embarrassed about having invisible internet friends to being pretty stoked that such a silly thread (which it became) was the garden that all these cool friendships have grown out of.
Another fond memory is the first meTa I ever posted, which was for a meetup because mathowie was coming to town. He brought mefi's own asavage with him (there are pix, it did happen). A ton of people showed up, we had a wicked fun night, and got a frjtz in-joke out of it.
If I start talking about all the awesome meetups I've been to (10th anniversary in PDX! Scotch meetup! Birding with jessamyn!), I'll never stop, so I'll collapse it all into one rather inadequate declaration that my fondest memories of this place are all about the excellent people I've met and become friends with. Thanks, Matt, for thinking this place up.