* Dia - (*nix) free tool, but limited icon sets, export to gif/jpeg etc.
* MS Visio - (MSWIN) propietary, but has nice stencils/icons
* MS Excel / OpenOffice Calc - Spreadsheet programs, good for producing server/rack diagrams
* OmniGraffle - (Apple) works well, can also use some MS Visio icon sets!
* xfig - (*nix) old, tedius, horrible-looking, but possible to persevere and produce nice diagrams
* (Paper/Pen OR Board/Marker) + Digital Camera - captures live sessions with digital images!