I’m a counter. Ever since I was about 12 and found out what digital roots were, I have remained obsessed with calculating them. To calculate a digital root, you add together all the digits in a number and then do the same to that result, until you’re left with a single-digit number. For example:
198,407
1+9+8+4+0+7 = 29
2+9=11
1+1=2
So, the digital root of 198,407 is 2.
It started with mailbox numbers, then phone numbers. 9 has a digital root of 0 (since 10 has a digital root of 1, and 9 is 1 less than 10, 9 has an effective digital root of zero). Thus, 3 and 9 are both magic numbers because 9s cancel themselves out when digital rooting, and 3 is 9’s square root. So, using this principle, the shortcut to the above example is:
198,407
cancel out the zeroes and nines and the numbers adding up to nine = 47 (because 8+1=9=0)
4+7=11
1+1=2. Same result, much more efficient calculation! /geekhat
I also picked up the habit of counting words and/or letters while reading (usually at the bottom of each page), and they have to come out to a multiple of 3.
I have a thing about evenness. If I rotate a pdf or tiff document at work, I have to overrotate it and rotate it back or else I’ll feel uneven and uncomfortable. If I step on a crack with my left foot, I have to step on the next one with my right foot (I try to avoid cracks, though). When I was a little kid, I remember having a thing about the colored tiles at the grocery store. 1 out of 9 were gray, and the rest were white. I had to step on the same number of gray tiles with each foot before I could leave the store! Another thing was stairs. I have to count stairs. In my childhood home (where I haven’t been since I was 18), I still remember there were 6 stairs leading up to my front porch, 3 more on the porch itself, and 15 from the first floor to the second floor (all great numbers, since they’re divisible by 3). There were 10 stairs leading down to the basement, and I hated going down there because of that. My last apartment had 14 stairs going up to the top floor, which bugged the crap out of me until I started counting the minor step up over the front door threshhold as 1. My current basement has 12 stairs, also good 
I’ve always had to wash and dry myself in the exact same way and the exact same order every time. I don’t really understand how other people can just casually rub their towel wherever until they’re dry, that would drive me nuts!
This all takes place inside my head though, so I don’t think anybody would know about it from pure observation. I don’t believe any of these things rise to the level of clinical OCD, since the behaviors don’t unduly disrupt my life. They’re just a part of who I am, I don’t let them bug anyone else, and that’s cool with me. I’m already a fast reader, so brief pauses to count the words in a sentence don’t slow me down significantly. And adding up numbers is actually fun. My issues aren’t *nearly *as burdensome as issues like excessive handwashing and checking locks or stove burners–those things have NEVER bothered me.