For 700c rims
Ever think of what you're riding or want to figure out a new ratio? Here's a nifty chart of gear inches and skid patches (for you fixed gearers). Whether you're thinking of an 18-speed road bike, 3-speed, or fixed/single speed, here's a good way to calculate it.
A good medium ratio should yield about 68-70 gear inches. Some of you like it a bit easier or harder, but this should be a nice starting point. If you want to ride fixed and plan on skidding, be conscious of choosing one that gives you more skid pathes (the amout of skid wears around your tire each time you lock up your rear wheel).
For instance, a 49 (chainring) x 18 (rear cog) will run a 73.5 gear inches while still allowing 18 skid patches around your tire. Respectively, a 38 x 14 ratio also runs 73.3 inches while only allowing 7 skid patches.