Dust off those old iBooks and Powerbooks and give them new life with Linux. Earlier this week, Ubuntu released version 7.04 of their desktop and server distribution, called "Feisty Fawn." While version 6 was the last officially supported version of Ubuntu for PPC, there is a community-supported PPC version of Feisty Fawn that you can download here. Ubuntu is a free (as in speech and beer) download.
Also, Yellow Dog Linux version 5.0.1 for PPC was made available in the Terra Soft store earlier this month, and the download mirrors will be availabe in a few weeks. For now, you can purchase YDL from Terra Soft in one of two bundles, priced at $99.95 and $49.95 respectively.