I needed to find some free tools for a client project where using Visio hasn't been practical. Here are three that have somewhat met my needs.
For UI mockups, I was able to get away with The Pencil Project, an add-in for Mozilla Firefox. I plan to see what it takes to create stencils, since, if it had flowcharting symbols, I could have done without a software listed later. I'd also like to create a better set of web shapes. All in all, I think this project has terrific potential.
I used MySql Workbench OSS (the free version) for database diagramming. This worked fine for me, even though I was diagramming for an MS SQL database, because all I cared about were the relationships and data types. And printing. Now, that part's kind of funny, because the OSS edition doesn't support printing directly. You have to export to PDF, then print that. Weird.
Finally, for a flow chart I needed, I settled on Diagram Designer. It's an odd program that does some things well, and others poorly. For instance, the lines really don't route around shapes properly compared to any other such program. But I really like the text editing, because, while a bit archaic, it's also very flexible. Then again, there doesn't appear to be a way to choose the default font. Why didn't I use Dia? Because I don't think it's very good, and doesn't seem to have improved much in the last four years. It always just feels hard to use. And Word is too klunky. And I can't justify installing Open Office a my client site (and its drawing module is kind of klunky, too.)