I've always just used OpenOffice Draw and it just occurred to me that there has to be something better out there.
So what do you use? Do you like it?
Bonus points for free things and for web apps.
I use Dia, the Diagram Editor.
Dia - Best Freeware Flowcharting (Visio) Tool
Dia is a free flowcharting tool, similar to Microsofts Visio. It is useful for building informal diagrams, database diagrams and database models, as well as drawing more complex charts such as UML diagrams.
Objects in the charts can be connected with lines and arrows in a variety of styles.
Object types in Dia include AADL, Assorted, BPMN, ChemEng, Chonogram, Cisco, Civil, Cybernetics, Electric, ER, Flowchart, FS, Gane and Sarson, GRAFCET, Jigsaw, Ladder, Logic, Map, isometric, MSE, Network, Pneumatic / Hydrualic, RE-i*, RE-Jackson, RE_kaos, SADT/IDEFO, SDL, Sybase and UML.
Dia saves diagrams in several formats, including XML, EPS, SVG, XFIG and PNG.
Diagrams in Dia can span multiple pages. For a few examples of diagrams created in Dia, see http://live.gnome.org/Dia/Examples. Dia is freeware and open source.