Interesting topic! I use the following -
Dropbox - as you mentioned - I have been using Dropbox since it was in beta and received the invite from the founder.
Freshbooks - Tracking expenses, invoices, time tracking - http://www.freshbooks.com
Asana - Project management and tasks - http://www.asana.com
Evernote - Notes - http://www.evernote.com
Google Calendar/Gmail
Wunderlist is pretty good on Mac - http://www.wunderlist.com
Jason