Unity is designed by Canonical, and is scheduled to become Ubuntu's default interface in the next release.
Just as Plasma Netbook is an alternative view of KDE, so Unity is a new front end for GNOME. Consequently, although designed with netbook's reduced screen size in mind, it is not particularly lightweight.
Unity reduces the panel to a minimum of applets, and places the menu on the left side of the screen in a launcher that also doubles as a taskbar and a navigator for the home directory. Top-level menu items are also displayed on the desktop. The arrangement of icons on both the panel and the launcher shows an emphasis on light, modern computing, emphasizing such activities as social networking listening to music.
Unity is still in development. Currently, however, one of its main weaknesses is a lack of organization in the launcher, which perhaps is required to do too many things. Another is a tendency to bury customization options so far down in the interface that many users are apt to miss them. At this stage, though, it is hard to make any recommendations about Unity, since its design is apparently still in flux.
The developers will probably not thank me for the description, but I always think of Xfce as what GNOME would be if designed with traditional Unix principles in mind, and kept light and efficient.Or, to put it another way, Xfce manages to balance ease of use with speed and a small footprint compared to GNOME or KDE.
This balance makes Xfce ideal for netbooks, particularly older ones with less than a megabyte of RAM, or new ones in which hard drive space is scarce. Another advantage is that, unlike some of the recent netbook-specific designs, Xfce's interface is standard enough to be instantly familiar. In fact, you might consider running it on your workstation as well.
If these interfaces are not enough for you, another alternative is to fall back on the window managers that were GNU/Linux's first interfaces. Window managers such as FVWM, Enlightenment, and Fluxbox are still popular, especially among more advanced users, and may be all you need for your netbook. The larger distributions will include most of them, and some smaller ones will use them as the main interfaces, so they are easy to experiment with. You could even choose a tiled window manager such as Ratpoison, although I would not recommend one for anybody except an advanced user.
Exactly which interface you choose will depend on your priorities and preferences. Personally, I prefer Plasma Netbook -- but, then, I am mainly a workstation user. If you value speed, LXDE might be the choice for you, while if you use net applications, than Chrome OS might be ideal. But, whatever you are looking for in a netbook interface, the chances are that the free software alternatives will include at least one that will satisfy your needs.