Yes. You can install Linux on Intel-based Macs using the same discs that you would use on a PC. You can either install through Boot Camp, or through booting from the CD.
On older PowerPC Macs, it can sometimes be trickier. Ubuntu, the most popular distro, no longer supports PowerPC processors officially. However, Fedora, OpenSUSE, and Debian will still work. A custom Debian installation is probably the best idea for a G3 iMac; Fedora and OpenSUSE should be fine on a G4 or G5.
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There are numerous options for installing Linux on a G3 iMac. One such option would be MkLinux (See links below). The Low End Mac website maintains a list of links about Linux on Macs.