High school. She hung out with my guy friend, and we became really close.
Here is where I met the people any of whom, depending on context, I would call my best friend:
[ol]
[li]5th grade, band practice (I played trombone, he played clarinet)[/li][li]6th grade, YMCA soccer league (didn’t become friends until high school, though)[/li][li]Freshman year of college, sidewalk in front of women’s dorm (though I’d been emailing her for weeks – recognized her from her Baby Book photo – hey, I guess that was a proto-“Facebook”)[/li][li]Sophomore year, college writing center (he was a fellow tutor, later we were roommates)[/li][li]Grad school, university classroom (now married)[/li][/ol]
Gym class in high school playing hacky sack.
I met my best friend at work, in Cubeville. Our cubes shared a back wall so we could hear each other talk. I will really miss seeing her face-to-face when I finally do move to Boston.
Cannot for the life of me remember. It was ten years ago. Or eleven.
I may be weird.
Online. I’ve yet to meet her but we’re pretty close – as we should be if I’m talking about her in a thread about best friends – and have plans to spend a few weeks together in Australia in a couple years.
My best friend offline I met at work when he and another friend were talking music and I butted in. He, his girlfriend, some other friends, and I all went to Bonnaroo earlier this year and I talk to him pretty often, though mostly at work 'cause I’m not really the type to hang out much and we live about twenty miles from each other.
First day of college. She bounced into the room at the professor’s house/temporary residence, wearing, I think, bright pink or orange, and said something very enthusiastic and friendly to me, her new goth housemate. Seven years ago.
She still wears a lot of fuschia. And orange. And lime green.
I really wish she didn’t live eight hours away.
The SDMB, or university. Depending on how you look at it.
NailBunny: I LOVED Whisper, the Winged Unicorn when I was a kid! I wonder if I still have my copy, or if my mom does.
I met my three closest friends at: my local comic book shop; a screening of The Rocky Horror Picture Show; and the first day of freshman orientation in college.
My best friend and I either met in high school in 1990 or 1991, or at the local coffee shop or possibly a mutual friend’s house sometime between then and summer of 1993. The actual first meeting has been lost to time as we both were doing massive quantities of drugs at the time and didn’t really become great friends until summer 1993. The fact that we don’t know when or how we met has become a joke now.
I don’t mean this as a snark, but: do adults have “best” friends?
I have several very close friends (the kind like family), and others I’d call good friends (the kind you hang out with, bitch to, travel with sometimes, etc.). Picking one and labeling him or her “best” seems juvenile to me – unless we’re talking spouses.
Junior year of high school, new student orientation day, playing chess in the library.
Martial arts class.
I met him at work. He’s like the brother I never wanted. 
Though it is nice to find a friend that I know 10 years from now he will still be there. I never had that comfort with any other friendship before. To be accepted for who I am, with the millions of faults and all.
Apparently we do.
I don’t think it is juvenile at all… and I know a lot of people that are married, who wouldn’t consider their spouse their “best friend”.
Just speaking for myself here, but when I say best friend I don’t mean it as a value judgment really, there are just some friends I’m a lot closer to than others. And I do have more than one “best friend,” a lot of people do. All it really means is that I have greater relationship with them than some of the others. They’re family.
Now when we start competing for and withholding the title for stupid transgressions, then it’s juvenile. Right now it’s just a term which has carried over.
I have 3 best friends (I know, that doesnt make sense, but it’s true)
BFF (best female friend): Thru work in Kansas. She was my boss. She’s a lot like me (she has a fear of the number 5, btw, & she’s the one who taught me to embrace my whimsicle nature)
BMF (best male friend): through a free online dating site. I was interested in gaming, not dating, and we clicked asap - I consider him the male version of my bff
Bestest of the best - met him on the same site as my bmf, but we met in person at a bookstore over a coffee (of course!). The little sneak walks as silent as a mouse & snuck up behind me, scaring the b-hay-suess out of me… we clicked, and I wasnt even interested in dating him (or anyone) when we first started to talk - but I thought he’d be a good friend to converse with.
I was his after the first kiss (which wasnt on the first date). Does that make me easy if it wasnt planned? 
7th grade, Wheelersburg, Ohio, 1960
I opened a bottle of wine for her in the kitchen at a friend’s birthday party. Michael Stipe had just through the room and looked at me suspiciously. The not yet then Ms. Kuboydal pointed a bottle of cheap red wine at me and asked me if I could open it. I swear she had a nimbus. I couldn’t say a word but I opened the bottle and returned it. The next time I saw her she jumped into a kiddie pool in my backyard. This time I started talking.