With my last client we used to have a spreadsheet on Google Docs. Not at all secure but people weren't putting bank passwords in that either. More like test logins to various WP sites we had and stuff.
We used to have a spreadsheet with a silly password that could be hacked in 1 minute using rainbow tables.
Now we moved to a shared Google spreadsheet. Not really that much more secure, but at least it's easier to manage.