Racial ability: "Preparation".

Mutare Drake, when you amass hundreds of peasants, especially with Elaine, you get a rather significant daily income boost. Before patch 1.5, I used to save them up and then convert them all into crossbowmen at once. Now, I either turn monks into knights when the final battle is in an open field (usually against a human player in HotS), or I turn swordsmen into monks (usually against the AI). Also, in versions 3.x, improved peasants also generate income.