alternative leavening agents other than baking powder, and their ratios of substitution

I must confess, I can't give you straight substitution amounts -- all of these leavening agents behave somewhat differently, so in most cases a straight substitution for baking soda isn't appropriate.

Ye Olde List of Leavening Agents:

Biological Cultures (Wikipedia, plus kefir whey I've seen recipes for)

• Yeast - norm is 1 tsp/pound flour for 1-2 hour rise time
• Sourdough - uses 1/2 to 1/4 of final flour mass, rise time varies
• Kefir whey - similar to sourdough, unknown rise time or measurement
• Buttermilk - no clue what substitution ratio, but wikipedia lists it. Probably acts similar to kefir whey and sourdough.
• Yogurt -- wikipedia lists this as a leavening agent too, but I'm skeptical.
• Beer (unpasteurized, with live yeasts)
• Ginger beer (same deal)

All of these share the common ability to ferment flour, when handled properly.

Chemical Leaveners (Pulled straight from On Food and Cooking)

• Pearlash and potash (mostly potassium carbonate). Purely historical leaveners, these were derived from wood ashes, and were the predecessors to baking soda.
• Hartshorn AKA baker's ammonia (mix of ammonium carbamate and ammonium carbonate). Not used much anymore. It was once made from deer antlers, ergo the name. When heated, this produces both ammonia and carbon dioxide. Because of the ammonia, this cannot be used as a straight substitute for baking powder, due to the changes in flavor and smell it causes.
• Sodium bicarbonate (baking soda), basic component of baking powder, reacts immediately with acid
• Cream of tartar (tartaric acid) -- Acidic component, reacts with bases immediately
• Monocalcium phosphate -- Acidic component, common in baking soda, reacts immediately
• Sodium acid pyrophosphate -- Acidic component, reacts slowly with base after mixing
• Sodium aluminum sulfate -- Acidic component, provides the "double" in most double-acting baking powders. Reacts slowly under heat.
• Sodium aluminum phosphate -- Acidic component of double-acting baking powder, acts during early cooking at 100-104F/38-40C
• Dimagnesium phosphate -- Acidic component of double-acting baking powder, reacts early in baking @ 104-111F/40-44C
• Dicalcium phosphate dihydrate -- Acidic component of double-acting baking powder, reacts late in baking @ 135-140F/57-60C

So, basically you need an acid + baking soda, or one of the heat-activated compounds (hartshorn, or one of the last four salts). In many cases a dough can provide its own lactic acid.

You can make a straight substitute 1:1 for baking powder by making your own, combine 2 parts cream of tartar and 1 part baking soda. This is essentially the same thing as baking powder, which consists of baking soda + acid.

Baking powder is used in recipes where there is insufficient acidity in the rest of the ingredients to activate the baking soda.

This type of leavening is completely different from biological leavening, and there is no reasonable substitution to be made (i.e. yeast).