The Ubuntu netboot (network) installers are fantastic, and for how good they are, they are hardly ever publicized.
The idea of just downloading your OS whenever and wherever you need to install it is kind of astounding. It makes the Windows/Mac experience of having to dig up some DVD and type in a 25-digit number seem positively archaic. It's the difference between walking across the street to buy bottled water, and turning on the tap and just having water appear. It completely changes the economics of the game when you no longer have to think of your operating system as a scarce resource.
As I understand it, there are three ways to do a netboot install. I've previously written about installing Ubuntu by booting from files downloaded to your hard disk. It's also easy to do a netboot install from either a CD or a USB key, and the procedures are very similar.
All three methods are nice because they have small initial downloads, about 10 MB; they then download the rest of your OS (and only the latest version of each package) at install time. It's a waste of time to download a ~700MB CD image if you're going to upgrade half of your packages right after installation. (Software is usually out of date by the time you install it! Especially during the development period for each release.) The installer is small enough that you can even burn it to one of those business card CDs.
The netboot installers are also versatile. They will install, at your request, any (or all!) of the following: Ubuntu desktop, Ubuntu server, Kubuntu desktop, Xubuntu desktop, Edubuntu desktop, and many more. They also support installing to encrypted LVM for full-disk encryption. (The netboot installers are based on the ncurses Debian installer.)
The hard disk method has the advantage that, of course, you don't need to use any external media. However, in my experience, the CD and USB key installers are a little less flaky. Unlike a hard disk installation, they also work even if you don't have Grub already installed.
For either the CD or USB key methods, you can find the appropriate files here:
(This is for Jaunty/i386. If you want a different release or have a different architecture, like amd64, adjust the URL accordingly.)
For CD installation: download mini.iso and burn it to your CD.
For USB media installation: download boot.img.gz and follow the instructions here. It will boil down to doing something like this:
# zcat boot.img.gz > /dev/sdX1
# aptitude install mbr
# install-mbr /dev/sdX
Then boot from your new media into the installer.