Dia is a free open-source program that provides a lot of the functionality found in Microsoft Visio. It can be used to create UML diagrams, flowcharts, entity relationship diagrams, network diagrams as well as just about any other type of diagram you will need. It even allows you to create custom shapes for use in your diagrams. If you're not too excited about shelling out the money for Visio, give Dia a look. It runs on Windows, Unix, and Linux machines (folks on Macs are out of luck at this point, but it is an open source project so look for someone to port it at some point).