http://www.songkick.com might be able to help a bit. You can use it both to find info about shows you may have attended, and also to keep a record of those shows for you, so when this thread comes around again you'll have a nice list to reference.
My tally at the moment is 230 concerts, so I won't list them all here; you can see them at
http://www.songkick.com/users/skyrefuge/gigography In terms of "concerts I've written down", I think I win the thread so far, except that includes plenty of non-metal.
Here's a writeup I did on Songkick for the PMX:2 board:
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http://www.songkick.com/
A website that serves as both a historical record of concert performances, and source for upcoming concert information. For example, you can find out how many times your favorite band played in your city before you even knew about them. Or just see who's coming to town, Pollstar-style.
Then, when you sign up, it can serve as a personal database for concerts you've attended, and you can add/edit concerts, both upcoming, and past (adding photos, setlists, reviews, etc.)
Although I have a custom database for my music collection and listening statistics, I've never created a similar one for my concert tracking/history. Instead, I've only had that in a spreadsheet. So, for kicks I spent the last couple weeks adding my entire concert history to Songkick (which also meant that I created many of concerts for the older shows).
For upcoming concert notifications, you can upload your iTunes database or last.fm profile, and it will automatically "track" all of the artists in your collection. Unfortunately, it seems like half the shows I go to these days are for artists whose music I don't even own, so it won't tell me about those shows (though on the page for each show, it will show "similar concerts" coming up). You can also "track" venues.
The first day after I signed up I was informed about two new concerts (Alcest and Del Castillo) before I found out about them through my usual sources, so I think it's worth it just for that.
last.fm has a similar database, and a much larger population of users, so why did I tie myself to Songkick instead? I dunno, I just liked the feel better. It's focused solely on concerts, and they seem more dedicated to becoming the "wikipedia of concerts", so maybe over time they'll get more popular. They also seem to have better tools for analyzing and manipulating your data, and make it much easier to show off (which was my main goal anyway!) For example, you can see who has seen the most concerts by a certain band, or at a certain venue. I learned that the Metro is my most-visited venue (26 times), and that I've been to 217 concerts in all, both of which would have been tough to figure out from my spreadsheet.
You can also track other users, so if anyone wants to keep track of which awesome Chicago-area shows I'll be going to (or if you just want to know when is a good time to burgle my house), I'm at
http://www.songkick.com/users/skyrefuge
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The lesson to all the youngsters out there: whether it's on a yellow legal pad, a Google Spreadsheet, or in Songkick, WRITE THAT SHIT DOWN! You'll thank yourself later when you're old and your memory is broken.
Neil