PowerPCDownloads
Since 7.04 Ubuntu, PowerPC downloads of Ubuntu are not available on all Ubuntu mirrors. They have been moved into the ports section of the main mirror.
Daily build of the current Ubuntu development release
At present, the current Ubuntu development release is 12.04 Precise Pangolin. These iso's are available for testing:
Ubuntu
Alternate CD - http://cdimage.ubuntu.com/daily/current/
The Ubuntu Live PowerPC CD image is currently above 700 MB when fully downloaded and cannot be burned onto a CD.
Lubuntu
Live CD - http://cdimage.ubuntu.com/lubuntu/daily-live/current/
Alternate CD - http://cdimage.ubuntu.com/lubuntu/daily/current/
Kubuntu
Live CD - http://cdimage.ubuntu.com/kubuntu/daily-live/current/
Alternate CD - http://cdimage.ubuntu.com/kubuntu/daily/current/
Netboot
11.04 Natty Narwhal and 11.10 Oneiric Ocelot
For 11.04 and 11.10 use the minimal CDs and follow instructions in this post.
10.10 Maverick Meerkat
Ubuntu
The standard PPC CD image is above 700 MB when fully downloaded and cannot be burned onto a CD.
Kubuntu, Xubuntu, Lubuntu, etc
It is also possible to install by using the mini iso and then installing either the kubuntu-desktop, lubuntu-desktop, or xubuntu-desktop packages. See instructions in this post.
10.04 Lucid Lynx LTS
Ubuntu
Desktop live install CD, server install CD, server install image, alternate install CD, and netbook live install image for various platforms
Kubuntu, Xubuntu, Lubuntu etc
Kubuntu http://cdimage.ubuntu.com/kubuntu/ports/releases/lucid/release/
Xubuntu http://cdimage.ubuntu.com/xubuntu/ports/releases/lucid/release/
It is also possible to install by using the mini iso and then installing either the kubuntu-desktop, lubuntu-desktop, or xubuntu-desktop packages. See instructions in this post.
8.04 Hardy Heron LTS
Ubuntu
Desktop (Live cd), Alternate (text mode installer), and server cd:
Kubuntu, Edubuntu
Kubuntu http://cdimage.ubuntu.com/kubuntu/ports/releases/hardy/release/
Edubuntu http://cdimage.ubuntu.com/edubuntu/ports/releases/hardy/release/