Google Docs for informal stuff and collaboration, but Microsoft Office all the way for formal documents. MS Word can't be beat when you want to make a document look really great.
I can't afford multiple copies of MS Office, though, so I have OpenOffice installed on my laptop. +Gabriel Walsh
thanks for mentioning LibreOffice. I had never heard of it before, but I'll install it now.
For the Google Docs users who love LaTeX, you can use docs.latexlab.org
. You can make LaTeX docs, and save the results in Google Docs, so you can collaborate with others as well.