Package: ThingLab (much more than a UI) - allows you to play with constraint based models (like electrical circuits, geometery...) in a prototype based browser.
The UI of the Oberon language system - a rather different approach with "tiled autoresized/autorepositioned usually-nonoverlapping windows"
ACME for Plan 9 by Rob Pike was inspired by Oberon and mixes the Unix text interface and windows nicely. A reimplementation of ACME for Unix is available as Wily.
Various ideas were raised about providing something analog to X workspaces by allowing even easier access to switching projects, or a Win95-like taskbar. If you use Windows and don't know what Workspaces are, go to goScreen
Pad++ is a different direction emphisizing zooming and with lenses acting as concrete filters that change the way something is displayed. Think of moving an InspectionLense over different objects...
Now called Piccolo (after an incarnation as Jazz (Zoomable User Interfaces))
Fisheye techniques as in DateLens - meaning that some windows/panes automagically grow (and others shrink), allowing you to see more of what interests you.
