He's looking for a free version that uses very little memory.
Dillo and Links use about the least amount of memory I've seen, but unfortunatly, they aren't very feature rich.
You've hit on all the major linux browsers, so I cant think of anything. Why not just pay the $30 if you don't like the ads in Opera? You seem to enjoy Opera a lot.
Also, Konqueror has a bunch of controls for tweaking memory usage. Have you checked them out?