The best ones are actually all plastic water containers with a red plastic strainer lid in the shape of a funnel with a small hole at the bottom. Fly walk in and can't figure how to walk out.
The container is 1/3 filled with water and attached to a fence 20 - 30 feet away from your nose - down wind! You add one teaspoon of this brown powder that lasts a season and smells like sh*t. You don't want to be around it or downwind of this thing! It catches flies like you simply wouldn't believe.
So the upside is its effective, very low cost, takes minimal space, works with water an a safe chemical, is set and forget (empty the dead flies as fertiliser every month or so for your garden) produces a by-product that is good garden fertiliser and requires no betteries nor produces any light to affect your astronomy. The down side is your neighbours downwind will be wondering WTF just died - which is another good thing if you are fighting with some annoying neighbours.
I think national geographic sell these things for $29 with a year or more's supply of bait. EFEKTO Fly Bait traps it says on the label.
http://www.premaorganics.com/fly.htm