There are many improvements and new features since Privoxy 3.0.6, the last stable release:
Two new actions server-header-tagger and client-header-tagger that can be used to create arbitrary "tags" based on client and server headers. These "tags" can then subsequently be used to control the other actions used for the current request, greatly increasing Privoxy's flexibility and selectivity. See tag patterns for more information on tags.
Header filtering is done with dedicated header filters now. As a result the actions "filter-client-headers" and "filter-server-headers" that were introduced with Privoxy 3.0.5 to apply content filters to the headers have been removed. See the new actions server-header-filter and client-header-filter for details.
There are four new options for the main config file:
allow-cgi-request-crunching which allows requests for Privoxy's internal CGI pages to be blocked, redirected or (un)trusted like ordinary requests.
split-large-forms that will work around a browser bug that caused IE6 and IE7 to ignore the Submit button on the Privoxy's edit-actions-for-url CGI page.
accept-intercepted-requests which allows to combine Privoxy with any packet filter to create an intercepting proxy for HTTP/1.1 requests (and for HTTP/1.0 requests with Host header set). This means clients can be forced to use Privoxy even if their proxy settings are configured differently.
templdir to designate an alternate location for Privoxy's locally customized CGI templates so that these are not overwritten during upgrades.
A new command line option --pre-chroot-nslookup hostname to initialize the resolver library before chroot'ing. On some systems this reduces the number of files that must be copied into the chroot tree. (Patch provided by Stephen Gildea)
The forward-override action allows changing of the forwarding settings through the actions files. Combined with tags, this allows to choose the forwarder based on client headers like the User-Agent, or the request origin.
The redirect action can now use regular expression substitutions against the original URL.
zlib support is now available as a compile time option to filter compressed content. Patch provided by Wil Mahan.
Improve various filters, and add new ones.
Include support for RFC 3253 so that Subversion works with Privoxy. Patch provided by Petr Kadlec.
Logging can be completely turned off by not specifying a logfile directive.
A number of improvements to Privoxy's internal CGI pages, including the use of favicons for error and control pages.
Many bugfixes, memory leaks addressed, code improvements, and logging improvements.
For a more detailed list of changes please have a look at the ChangeLog.
A quick list of things to be aware of before upgrading from earlier versions of Privoxy:
The recommended way to upgrade Privoxy is to backup your old configuration files, install the new ones, verify that Privoxy is working correctly and finally merge back your changes using diff and maybe patch.
There are a number of new features in each Privoxy release and most of them have to be explicitly enabled in the configuration files. Old configuration files obviously don't do that and due to syntax changes using old configuration files with a new Privoxy isn't always possible anyway.
Note that some installers remove earlier versions completely, including configuration files, therefore you should really save any important configuration files!
On the other hand, other installers don't overwrite existing configuration files, thinking you will want to do that yourself.
standard.action now only includes the enabled actions. Not all actions as before.
In the default configuration only fatal errors are logged now. You can change that in the debug section of the configuration file. You may also want to enable more verbose logging until you verified that the new Privoxy version is working as expected.
Three other config file settings are now off by default: enable-remote-toggle, enable-remote-http-toggle, and enable-edit-actions. If you use or want these, you will need to explicitly enable them, and be aware of the security issues involved.
The "filter-client-headers" and "filter-server-headers" actions that were introduced with Privoxy 3.0.5 to apply content filters to the headers have been removed and replaced with new actions. See the What's New section above.