One thing that's become a necessity for me is the ability to connect to multiple Google sessions at the same time. Initially I could check my multiple GMAIL accounts (I have one for business, one for personal, one for my Body Corporate affairs) through something like Thunderbird, which does the trick for email, but when I need to access multiple online documents (each account has their own list of appropriate documents online) it becomes a little trickier.
A simple way to access multiple documents is to employ multiple browsers - I use CHROME for one, FIREFOX for another, and IE for the third. This is helpful to quickly identify which account is being used for whatever purpose and you can be online with all three accounts at the same time but for obvious reasons it becomes a schlepp and is pointless if you're using a machine which isn't installed with all the browsers.
Google's obviously become aware of multiple account users and they've introduced Google's Multiple Sessions. At the moment this isn't a complete solution to the dilemma of multiple GOOGLE DOCS accounts, but there are ways around most niggles (like sharing without notifying etc. so one logged in account can do the manipulation of multiple accounts' documents, but that is also a hack).
For now Google's Multiple Sessions is not going to revolutionise my ultra-busy and complex life, but it certainly is a feature to keep an eye on: you can access multiple GMAIL and CALENDAR accounts at once which is certainly very helpful. One anticipates that this feature will be refined over time and eventually replace the little hacks the rest of us employ to achieve multiple access on one PC. Let's wait and see!