Alerts about activity on associated accounts
I find it quite difficult to track all stack exchange sites I participate in. I quite frequently participate on Stackoverflow, but occasionally in others as well. So I sometimes forget to check individual sites for activities.
What I'd like to be able to do is to have a kind of tabs at the top, where I would see tabs of all the stack exchange sites I registered into. If they'd have the envelope notification beside them it would be even better so I would be notified there's something for me there to check out.
This would definitely make my participation much easier (and I suppose others would agre with me on this as well) because I wouldn't have to remember all those obscure URLs to some beta sites. I could just go to Stackoverflow and access them all from there (or any other site for that matter).
Regarding usability: These tabs would most probably have to be completely on the top (above user/site top bar, because that one is already related to a particular site).