WinCVS

If you keep drupal or selected modules under version control with CVS, you can create patches via the WinCVS user interface.

Just right-click the file you edited and select "diff selection" (alternatives: the "diff selected"-icon on the toolbar, or the menu Query » diff selection). This brings up a "Diff settings" dialog box that allows one to select different options for the output. The resulting diff is output to the WinCVS-Console and can be copy-pasted and saved.

WinCVS also allows one to create patches via the command line.