Usable, free ISO file editor that runs on Windows?

There is no "free (libre) utility" that does all this.

You can use 7-Zip to extract ISO images, WinCDEmu to mount an ISO image and InfraRecorder rebuild and write the ISO image to disc.

7-Zip, WinCDEmu and InfraRecorder are free open source programs (which is pretty much as 'libre' as it gets :). Portable versions of 7-Zip and InfraRecorder are available.

I don't know of any free software that allows you to mount an ISO as a read/write drive. ImgBurn though is a free tool that will allow you to extract the contents of an ISO, then modify the files, then re-create the ISO.

UltraISO: On June 2009, PC User magazine Australia offered v9 full version to its readers.

Here are the registration details:

Name: PC User readers
Serial: 4BF9-8D1E-1786-30A8

Do take note that this registration details will NOT work on the trial installer that is downloadable from UltraISO's website, but the original version can still be found by googling for uiso93pes.exe.

TotalMounter is advertised as a free ISO editor, but I have no experience with it.

Folder2Iso does what it is named for and can be used instead of an ISO editor.