It is interesting to see how various online services react to multiple user accounts. This is especially important when you keep your work and personal data and life separate. Consider what email address is attached to an account, where messages containing data are sent, and what happens when people move on.
For example I have 3 Google accounts, one personal and two for two different startups. Google has done a pretty good job - you can login to all 3 in the web interface and switch between them as needed. Some of their products don't work (eg Analytics) but the most commonly used ones do.
On Android you can register all your accounts with the system and on the whole things work well, although G+ doesn't support multiple accounts. Google's apps on iOS don't seem to have multiple user support at all.
I'm currently dealing with Dropbox who seem completely oblivious to the problem. Their support is comical over the issue, and they will lose a paying customer the moment Google Drive works on Linux. Trello is a great tool, but it too suffers from the only one account problem.
For those of us implementing online services this is another thing to make sure we get right.