With a change in commute to a foldup bike plus train, I was in the market for a rucksack that was both water resistant and could safely house my Mac Book Pro 15". Finding something affordable was actually fairly hard ork. After a number of hours scouring the interweb I came accross the DHB Luggit 30L.
At £24 this is a bargain. Waist straps are detachable, and it's got a neat unfolding waterproof cover if it's really pouring - though I've been caught in the rain a couple of times without this on, and the inside has remained completely dry.