The Getting Things Done (GTD) productivity system has a place for everything, but you have to know where everything should go. Lifehacker alumnus Kyle Pott tweeted a checklist from GTD founder David Allen that helps determine where that just-arrived email belongs.
A reference sheet Allen sent out in his newsletter, graciously posted to Kyle's site, uses plain language to help you narrow down what you're thinking about something ("I might want to commit to this at any time in the future") and tells you where that should go in a prim-and-proper GTD system (Someday/Maybe list item). Not everybody practices canonical GTD, as with Gina's simplified GTD, but this list helps you determine, at least, where any item or document should go in a fully realised productivity setup.
GTD Checklist [Kyle Pott]