aewm is a minimalist window manager for X11. It has no nifty features, but is light on resources and extremely simple in appearance. It should eventually make a good reference implementation of the ICCCM. A few separate programs are included to handle running programs, switching between windows, etc.
AfterStep is a Window Manager for X which started by emulating the NeXTSTEP look and feel, but which has been significantly altered according to the requests of various users. Many adepts will tell you that NeXTSTEP is not only the most visually pleasant interface, but also one of the most functional and intuitive out there. AfterStep aims to incorporate the advantages of the NeXTSTEP interface, and add additional useful features.