What's the range on granny gears - the innermost, smallest front chainring?
The Mountain Tamer Quad lets you get down to a 16-tooth granny gear, although in the fourth position.
For the upper range, it depends on the crankset - if you limit it to only rings in the third position I suspect a 74BCD 30-tooth would be the largest commonly available. If you include the inner ring of a double crankset they can be as large as chainrings are made - 60-tooth rings can be purchased and you can get plans to machine your own chainrings up to 92 teeth.
Gearing is commonly described in Gear Inches and to clarify, having your chain in the smallest front ring and the biggest rear cog results in the smallest gear.
The smallest chainring I've seen on a 5-arm crankset is a 20T, although 24T is a lot more common.
Shimano had a line called "Megarange" that was 11-34T. I believe it's no longer manufactured but you can find them online sometimes.
SRAM has a cassette in the PG-850 line that is 11-32T that I believe is still available.
Shimano has a number of models that are 11-34T and one model that is 12-36T.
I believe the biggest you can get is 12-27T.
IMPORTANT NOTE: Not all front or especially rear derailleurs will be compatible with these large range rings and cassettes. Check with the manufacturer - most have spec and compatibility docs online.
Out of curiosity I used a gear calculator to figure out what the gear inches would be if you actually configured a bike and could ride 20x36 on 700c wheels...an incredibly small 14.7 inches. That means every pedal revolution you travel about 46 inches, just over half the circumference of the wheel.