For the last month and a little bit I am using MaganeWP service. It is very useful administration service if you need to manage several WordPress blogs. Till now I am using free version which supports up to 5 WordPress sites.
What I find useful in ManageWP:
1) It is very easy to setup and integrate to your blogs.
2) All sites in one dashboard.
3) Important notifications emailed about your sites (including waiting updates). So you event don’t need to check them every day.
4) One click upgrades for plugins,themes and maintenance\optimization tasks for all sites at once.
These 4 things I find the most useful in the free version. Of course, there is a huge bunch of extra features in paid versions, which maybe I will try in near future.
So, if you haven’t looked yet at ManageWP, I highly recommend to give it a try.