This post will be nothing like as detailed as my last one, but I have tried Pushnote and Dropbox.
Dropbox I probably won't use, because other sites already fulfil its functions. I use ADrive to store files, Google Docs (thing 9) to let other people work on them, and YouSendIt to share them -- especially when that means sending hi-resolution files of nineteenth-century pictures from the Haddon Library to publishers for reproduction.
Pushnote I have given myself an account for, but I feel awkward already at the thought of using it. Its purpose is to let one express an opinion of a website. I have opinions of websites, yes, and often tweet these, with links, drawing attention to the sites; but Pushnote is to let you comment on sites that your readers can already see for themselves, and it's rare that I'll be able to think of a comment in that situation that won't be self-evident. Also, very few of the names I input into the box on Pushnote's 'Find Friends' page turned out to be followers of the service.
Pushnote has not got much for me to join in with yet. I might come back to it later, if I hear things are otherwise.