I've been thinking a lot about how I keep track of people who I've met over the years. Systems like Facebook try to do that for you (hey! you're friends with!) but they neglect to capture some critical information.
The most salient points for me about connecting or reconnecting with someone usually revolve around the circumstances of how we first met, the reasons that there's still some ongoing connection, and the particular details of the last meeting. If I can hit each of those three perspectives on a relationship, chances are that it's easy to start up a new conversation and easy to call someone to mind when it's time to make a new introduction.
Alas, most personal address book software does a lousy job of keeping tabs on any of this. You can add a "related to" field in some systems, but that's not something that I've seen pulled out and turned into a full featured exploring tool. If you're lucky you get to enter some free text in some notes field, but even then you might not get the ability to search it on the fly. And heaven help you if you want to tag or group or categorize or classify people (for your own memory purposes) without fighting a system that generally is indifferent to the idea that people might belong to more than one group.
If I can dream, I'd like the simple app - perhaps just a thin front end to my address book - that flashes names in front of me and helps me quickly remember who's who and how I might know them. This is multiply true right now because I have a lot of dormant connections to people that I'd like to refresh. A smart address book, or a routine way of collecting that information in a dumb address book, should be able to help that process out.
UPDATE: It turns out that one of the recent updates to LinkedIn adds a "how did I meet you" question, with the ability to link to a person as the introducer. It's a good start.