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 apply arbitrary "tags" to each request's headers. These "tags" can then subsequently be used by other actions, greatly increasing Privoxy's flexibity and selectivity. See tag patterns for more on tags.
Header filtering can be 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 the content filters to the headers as, well have been removed again. See the new actions server-header-filter and client-header-filter.
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 will 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) so that explicitly setting the browser's proxy settings is not necessary.
templdir to designate an alternate location for Privoxy's own CGI templates to make sure any locally customized templates aren't overwritten during upgrades.
A new command line option --pre-chroot-nslookup hostname to intialize 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 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 for compressed documents.
Improve various filters, and add new ones.
Include support for RFC 3253 so that Subversion works with Privoxy.
Logging can be turned on or off.
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 improvments.
A quick list of things to be aware of before upgrading from earlier versions of Privoxy:
Some installers may remove earlier versions completely, including configuration files. Save any important configuration files!
On the other hand, other installers may not overwrite any existing configuration files, thinking you will want to do that. You may want to manually check your saved files against the newer versions to see if the improvements have merit, or whether there are new options that you may want to consider. There are a number of new features, but most won't be available unless these features are incorporated into your configuration somehow.
standard.action now only includes the enabled actions. Not all actions as before.
Logging is off by default now. If you need logging, it can be turned on in the config file.
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 the content filters to the headers as, well have been removed and replaced with new actions. See the What's New section above.