I am using two small fans from a computer to provide ventilation.
I'm glad you mentioned this. I was curious about this during the middle of summer.
I thought that by attaching the fan wires to some solar cells, it would work fairly well. Pumping the heat out during the day and then the fan would automatically shut off at night when the light was gone. Mr. Horn is correct about the bees getting in the fans so my plan was to cover the it in mess (window screen stuff). That would eliminate the casulties.
I never did do this, one because I thought it would be a pain in the butt when I needed to get into the hive or move supers around. The other, for the same reason that Mr. Horn & Mr. Bush said about earlier in this thread, the bees fan ventilate themselves. Still, I'm sure a single computer fan wouldn't make that much effect any how. Depends on the size I guess.
Ok, I've went on long enough about this. Take 'er easy.