Update October 2016: As mentioned here it is officially documented how to install WWB into Eclipse Neon (64 bit)! Great! Thanx WRI.
This is tested on Windows and Linux:
Download and install a 32bit version of Eclipse Kepler from http://www.eclipse.org/downloads/
On Linux x86_64 make sure to have 32bit Java installed and configured,
and, e.g. on Fedora do: ...
Wolfram Workbench is not available in the Eclipse Marketplace.
You can install the Wolfram Workbench Plugin for Eclipse using the instructions here.
Eclipse 4.6 (Neon) or later are supported (I personally have the plugin installed on Eclipse 4.9).