DEVELOPMENT
An Introduction into channel programming by Apple can you find here. The first three parts are already translated in German by Bodo Burtscher and you can download them here (part 1, pdf, ca. 400K), here (part 2, pdf ca. 480K) and here (part 3, pdf ca 140K).

A channel consists only of 5 components:

The logic, that is stored in an xml file, mostly /Channel/Channel.xml.
The icon, stored as icns file, mostly /Channel.icns.
The interface, stored in the known nib files (Interface Builder) under /Channel/de.lproj/Channel.nib (where "de" stands for German)
The name, saved as xml property "CHANNEL_NAME" in the file /Channel/de.lproj/LocalizedResources.plist
The description of the channel, stored as xml file, often /SherlockChannel.xml
These files can be edited with the Projectbuilder or Interface Builder. These tools are available at the Development Tools from Apple (without costs!).

In the logic file you can use the languages JavaScript and XQuery. Javascript is better for doing the logic and XQuery is very good for parsing an searching xml or html files/data.

A Sherlock mailing list from the Apple Developer Center can you find here.

Jaguar required