I will remember:
Where my Brain - the Psion, currently a 3MX - is, and to enter stuff I need to be reminded of in it. "I wrote it down so I wouldn't have to remember it!"
If I've been somewhere before.
What I did in bed with people. An ex never could - if sex were a meal, I could remember what we ate, while she could just remember how she felt afterwards.
The rules of games I haven't played in 25 years.
How to spell words. I'm a lot better at this now than I was then.
The films, plays and operas I've seen (nearly all of them anyway) and their basic plots.
Too much of various meetings.
Conversations, in real life or online.
Lots of details about 6502 / Z-80 assembly on 1970/80s home micros like the Atari 800s, plus x86 DOS.
Music. And lyrics, if I've sung them a couple of times. It can be quite hard to pin lyrics to the right song if there are no musical cues.
Faces - provided they don't change too much - and names. But...
I don't remember:
Which names go with which faces. At least not without using them while talking to them quite a few times. Matching those two to email addresses was bad enough. Having to remember their LJ user name too will make it worse. Once I've got it all right, it tends to stay got though.
Birthdays, unless they're in the Brain.
Ditto dates, times, places, meetings.
To dot all my i's when handwriting.
Who was in the films, plays and operas. Unless they were particularly good / bad.
What I did on a particular date. Here's where having a Brain doesn't help - it's too easy to wipe old info. It also means I don't have that many memories from childhood. Some, yes, but not nearly as many as some people do.
All the APIs on Windows. But then no-one can.
To post everything first time :)
[updated, as I remember things]