On the Jams page, it tells you what game jams you've created and submitted games for, butting would be super helpful to also have a section that shows what jams where you've clicked the "Join Jam" button so you don't lose track!
View Joined Game Jams
This is actually something I'm surprised wasn't there from the start. It's very easy for me (and I assume others) to join a game jam a week or month in advance eagerly, get caught up in work or other issues, and forget to add the jam to their plans when it sneaks up.
I think in addition to showing what jams you've currently joined, a quick "Add to Google Calendar/iCal" button by them, as well as a link to the Jam page of course, would be immensely helpful, as it'd give most people a quick way to add it to their schedule!