self-leveling concrete
here's some info about self-leveling concrete: http://www.quikrete.com/diy/PreparingSurfacesforCarpetResilientFlooringandTile.html.
it's basically a very watery concrete mixture designed to be poured onto the floor and flow freely until it finds its level and bond to the concrete underneath. i'm going to use this to level out all the unevenness of the concrete slab before doing any flooring.