With one pound of each of the following ingredients dissolved in water to make up a total volume of one gallon, you should expect the following specific gravities:
Dried malt extract - 1.042
Malt extract syrup - 1.036
Corn sugar - 1.036
Cane sugar - 1.042
Brown sugar - 1.042
Honey - 1.036
Brewery Grade Corn Syrup - 1.036
Rice Syrup Solids - 1.042
Brewer's Pale Malt - 1.025 - 1.030 (depending upon your efficiency!)
Munich malt - 1.022 - 1.027 (depending upon your efficiency!)
Wheat malt - 1.025 - 1.030 (depending upon your efficiency!)
Cara-pils/Dextrine malt - 1.024
Crystal malt - 1.015
Black malt/Chocolate malt - 1.010

Okay, how do you use this information? Easy! Simply multiply the last two digits on the gravity for each ingredient in your recipe by the number of pounds, add these amounts together and divide this total by the number of gallons.
For example: Let's say we have a pale ale recipe that calls for two 3.3 lbs. cans of light malt extract (hopped or unhopped makes no difference), a pound of crystal malt and a half pound of pale malt. What should we expect the original to be? First, let's multiply the weight in pounds of the malt extract times 1.036 (actually, let's drop the 1.0 part to make the arithmetic easier). That equals 237.6. Let's add this number to the value from the crystal malt (1 X 15 =15) and the value from the pale malt (1/2 X 25 = 12.5). Altogether that is 237.6 + 15.0 + 12.5 = 265.1. Divide this sum by the number of gallons in this recipe (5) and you get 53.02. This means that the original gravity should be approximately 1.053. These same ingredients brewed into a six gallon recipe should yield a brew with an original gravity around 1.044(265/6 = 44 or, as we would say, 1.044). On the other hand these same ingredients would produce a beer with an O.G. of about 1.066 if brewed in only four gallons (265/4 = 66 or 1.066). These numbers are only approximations, but they can be quite helpful when trying to formulate recipes. Obviously, as more refined and processed ingredients (e.g. malt extract, sugars, honey) are replaced with less refined ingredients (grains), then these numbers are much more variable. The yields will greatly depend upon your efficiency in the mashing and sparging processes. Still this chart should prove to be a helpful tool.