Spacefinder from the Chicago Reader where you enter your zip code and cost range, then Z-I-N-G, Z-A-M, B-O-O-M! You've got all your ads together in one page. Very cool.
This website is done in conjunction with "The Reader" the newspaper that is prevalent in the North Side. This paper comes out every thursday. It's got the best listings of apartments for the North Side. Better than the wimpy Tribune.