I had the same concerns a few months ago during a spring heat wave that put an IPA I was fermenting in my living room over 80F. Run a Google search for "Son of Fermentation Chiller" and you should be able to locate some schematics for a great solution.
Basically, you will be building a box out of 2 inch extruded polystyrene insulation (i.e. "Pink Board"). It has two compartments: 1 in the front for your fermenter, and 1 in the back for some ice. The two are connected through some small openings at the top, and temperature is controlled by a computer's 30mm cooling fan hooked up to a standard AC thermostat.
It runs on 12V AC to DC (adapter from radio shack) so it's very friendly on the ConEd bill. A bit of an eye-sore, but my mother-in-law came through in the clutch with a quilted cover that gives me some added insulation and disguises the box so as not to offend my wife's delicate sensibilities.
I rotate through 1.5L bottles of ice 2 at a time about once or twice a day, depending on the difference between ambient temp and fermentation temp. But I've generally been able to hold it down as low as the high 50s lately.
Last edited by AstoriaBrewer on Wed Jul 07, 2010 1:56 pm; edited 2 times in total (Reason for editing : typo)