Ubuntu Netbook is not lighter. It comes with a panel optimized for smaller displays (saves place since the window title, etc will go there) and so on. Also includes netbook stuff (like it works perfectly with eeePC out-of-the-box).
Alternatives, for a more lightweight operating system:
- Crunchbang Linux. Uses lightweight Openbox window manager, lxpanel as panel. Simple, not that hard to use (I would say it's easy). REALLY light.
- Lubuntu. It's in beta, uses LXDE as main desktop environment. Honestly, I wouldn't install it, even the 9.10 alpha have never got finished (I'm part of the LXDE team and I follow the development really closely.)
- Xubuntu. Uses XFCE environment, kinda light and not. How can I say.. sometimes it uses more memory than Gnome which is kinda funny. XFCE is out off development for a while now, doesn't seem to be too active (don't bite me for this but seriously it seems to be dead.)
U-lite (never tried it personally). It claims to be lightweight. Seems to be using GDM+LXDE combination. Strange for sure.
Minimal disc. These discs weight about 10mb. A simple ISO. You have to boot it, install everything from the basics. You can build lightweight environments then with this method. However, it requires some experience if something goes FUBAR.... :P
Other alternatives are mostly based on Debian or other distros. (Also you could use Lucid Puppy ? Or maybe its for slower machines...). Like Zenwalk. For this, you can use Distrowatch's search tool. Pick your choice, check the distro.
In my opinion you could try using "Little Susie". It's a hungarian lightweight distro, built for old PCs, using openSUSE and SUSE Enterprise base. Comes with optimised, really-fast, stable KDE3. (Hope it comes in english too... honestly I don't know. If there is no such version, just comment and I'll message the main developer, project founder.) Or just use Kubuntu's KDE3 remix. Here it is.