Privoxy, as an heir to Junkbuster, is a Free Software project and the code is licensed under the GNU General Public License version 2. As such, Privoxy development is potentially open to anyone who has the time, knowledge, and desire to contribute in any capacity. Our goals are simply to continue the mission, to improve Privoxy, and to make it available to as wide an audience as possible.
One does not have to be a programmer to contribute. Packaging, testing, documenting and porting, are all important jobs as well.
The first step is to join the privoxy-devel mailing list. You can submit your ideas, or even better patches. Patches are best submitted to the Sourceforge tracker set up for this purpose, but can be sent to the list for review too.
You will also need to have a cvs package installed, which will entail having ssh installed as well (which seems to be a requirement of SourceForge), in order to access the cvs repository. Having the GNU build tools is also going to be important (particularly, autoconf and gmake).
For the time being (read, this section is under construction), you can also refer to the extensive comments in the source code. In fact, reading the code is recommended in any case.