If you develop your own nodes, please also pay attention to the KNIME Noding Guidlines, which specify how KNIME nodes should behave.
The preferred way of developing for KNIME is to download the KNIME Target Platform definition, which is provided on the download page, and use it your own Eclipse installation. Consult the Eclipse documentation on how to use target platforms.
You may also want to have a look at the KNIME Node Wizard that can be installed from the KNIME Update Site (look in the KNIME Node Development Tools category).
We also provided a KNIME Software Development Kit (SDK) which is an Eclipse SDK with some KNIME plug-ins already installed. However, using a target platform definition is the preferred way of developing KNIME extensions.
If you want to browse KNIME's source code, the easiest approach is to include source in your target platform or alternatively install the source features into your SDK.
When using a target platfrom you can simply select the option "Include source if available" in the dialog where you select the target platform's contents from the update site (see screenshot).
If you want to install the sources directly into your SDK (which we don't recommend), they are available on the standard update site in the Sources category.
Once you add a dependency to a KNIME plug-in in your own plug-in you can browse the source code from within Eclipse.