Now that I'm participating in 3 stackexchange 2.0 sites, and the 3 original SO sites, it gets really hard to remember to visit every one of these sites to check for activity. If you factor in meta sites, that's an extra 4 sites I need to check. I know about email notifications, but for things such as comments, you don't get email but do get a notification.
I propose some kind of drop down on the top left which will show you the sites you are a member of. Some ideas for sorting the list are: based on reputation, alphabetical, or user-customizable. It would be very useful to see a notification icon (mail icon) next to each one too.
This is similar to, and would replace, my other feature request: Easy navigation between meta and "parent" site. It's also similar to Easy Navigation Between Stackoverflow Family-Sites, except that this is for all StackExchange sites.