This post will be brief but Visio is better and has the most capabilities to create some very high-end diagrams and flow charts. However, Visio is not always available as it's fairly expensive. So for those other times, an Internet connection and a Google Apps account (google.com/a) can fill in quite nicely. Via Google Drive > Create > Drawing and you're off to creating some also nice diagrams in a flash.
I'm curious how many domain owners use Google Analytics to see who's coming to their site. It's free to get started, simple setup that Google walks you through adding a custom DNS record they use to verify your ownership of the domain. Once in, the results are gorgeous. The metrics provided are pretty powerful also. They can be used to fix sections of the website that are not being seen. It also shows the behavior of user activity on your site. Another great Google App.