If you’re interested in the Cocoa API, there currently isn’t a list online. You’ll need to take a look at the header files that come with Espresso. Once you’ve right clicked the application and chosen “Show Package Contents” the header files are located in Contents/Headers.
The best way to figure out Sugars (be they Cocoa actions or syntax definitions) is to look through the wiki, and then examine what’s actually being done in the Sugars bundled with Espresso (located in Contents/SharedSupport/Sugars). The HTML and CSS Sugars tend to be the best internally documented. Obviously for Cocoa Sugars you’ll need to look at the example Sugar provided in the Wiki instead.