Although I routinely and compulsively change my window manager about every two weeks, I still think WindowMaker is one of the hypest window manager I've come across. Based on the NEXTSTEP GUI (from whence Apple's OS X interface also stems), it's flexible, fast, and it looks very nice. As you can see from the ps entry, it also has a very low CPU and memory footprint.
USER PID %CPU %MEM VSZ RSS TTY STAT START TIME COMMAND bjb 19772 0.3 0.7 4864 3036 tty1 S Oct30 11:47 /usr/bin/WindowMaker
You can download my WindowMaker theme here (650K). It's based on the Simple_Elegance theme found at www.themes.org.