Lovely Charts is a great online resource for creating a number of quality online flowcharts, network diagrams and the like. The free account allows you to save one project on the site. You can still create new projects and save/export them as much as you like. You can only save one though. If you update your account (for a very reasonable fee) you can save many projects and edit them online.
It's a very useful tool for those of us who rarely dabble in the art of network diagrams and flowcharts but sometimes need a graphic to support an idea to upper management. Let's face it, Visio just isn't worth it and there are few substitutes. This is a great one.