Is there a good, free software for creating ISO images from files or discs?
No shareware or "free" software with greatly limited functionality, please!
Edit: For Windows
I'm assuming from your answer that your OS is Windows. On that, I use InfraRecorder. It:
You may try Brasero If you run Linux.
If you are running Windows as OS then I recommend the lightweight ImgBurn is a powerful 100% free tool for burning CDs DVDs.
If you don't like or have already tried ImgBurn the there is another one CDBurnerXP. It is also pretty lightweight and satisfactory in looks.(If you don't like the ImgBurn's interface)
I personally don't create ISOs very much. If I have to create one, I use CDBurnerXP, which was already mentioned. For mounting I like to use Virtual CloneDrive. It's very lightweight, simple and integrates itself seamlessly into Windows.
If you're looking for a program that can create and mount ISOs I can recommend DAEMON Tools Lite.