For those of you who haven't heard about Gliffy, let that come to an end now.
Gliffy is a free online diagram creation tool built in Adobe Flash. This means its entirely cross-platform, which makes it easier for those multi-OS households and organizations to do development. No download is required (except possibly for Flash Player, if your copy is ridiculously out of date). Free accounts come with 3 private documents, and unlimited public documents. You can pay $30 for a year account, or $45 for two. This will net you unlimited private documents, priority support, and unlimited image uploads, as well as freedom from the occasional ads.
Editing in Gliffy is really easy; you have a menu on the left with categories of basic shapes, as well as standard icons for major diagramming applications (UML being the one I use the most, but there are also icons for computer networks, user interfaces and building floor plans). Just drag, drop, and customize. Text fields are built into objects where applicable, or you can add your own. Once you've lined things up the way you like, you can group clusters of objects together with a simple keyboard command (Ctrl-G). If the right shape or icon for your application doesn't exist, you can upload an image or build a new shape out existing ones, and group them.
Gliffy keeps track of your version history, so its easy to go back to an older version of a diagram if you added something that didn't work. There is also excellent integration into blogs and wikis (Confluence users, take notice!), and documents can be tagged, shared and collaborated upon.
For me, this tool has been incredibly helpful in my software engineering courses. I'll probably also wind up using it at work to create simple flow charts for how the new blog software we're implementing. It has to be, by far, the easiest tool for this kind of job I've run across.
POINT OF BALANCE: Like all applications, Gliffy has a few bugs. I've had a few scripts hang on me before, which either required a page refresh, or simply navigating back to the Gliffy login site, and logging in again. I would be cautious before trusting any critical data to ANY free online service, since it isn't your machine and you aren't paying anyone for the service. That said, if you just need to make a diagram, consider Gliffy.