It is possible to create custom stock firmware through the use of iPodWizard or iPodEditor.
Users are able to change images and screen titles through the use of such software. There are also modified firmwares that allow for the use of custom background images (aka bghack) and firmware that emulates the look of the 6/7G Classic (aka split view). One popular use of custom firmware involves making a dark theme for the iPod Video. Also thanks to firmware editing, there exist a hack to run pre-cracked clickwheel games on 5/5.5 Gen iPod Videos.
As of now, only iPod 1-5.5G, Nano 1G (2G?), and Mini 1-2G have stock firmware that the user can edit. Most notably, the iPod 6G/7G (Classic) has yet to be made modifiable due to having encrypted firmware.
To see a list of resources and downloads, visit the Software modding page.