K-Meleon mostly and then Chrome the rest of the time. K-Meleon is based on firefox's rendering engine, but it doesn't hog resources like firefox's GUI does so I use it. On Linux I use Galeon and the rest of the time Firefox. On my mobile I use Opera mini.
For fast starting of hogging web apps like gmail and facebook I do use prism by mozilla. No memory hog here, only uses 80MB of memory for basic operations and only loads on page at a time. Single load app, nothing frills or thrills here.