1. RefControl
Mentioned above, this extension allows more fine-grained referrer spoofing than Torbutton currently provides. It should break less sites than Torbutton's referrer spoofing option.

2. SafeCache
If you use Tor excessively, and rarely disable it, you probably want to install this extension to minimize the ability of sites to store long term identifiers in your cache. This extension applies same origin policy to the cache, so that elements are retrieved from the cache only if they are fetched from a document in the same origin domain as the cached element.

3. Better Privacy
Better Privacy is an excellent extension that protects you from cookies used by Flash applications, which often persist forever and are not clearable via normal Firefox "Private Data" clearing. Flash and all other plugins are disabled by Torbutton by default, but if you are interested in privacy, you may want this extension to allow you to inspect and automatically clear your Flash cookies for your Non-Tor usage.

4. AdBlock Plus
AdBlock Plus is an excellent addon for removing annoying, privacy-invading, and malware-distributing advertisements from the web. It provides subscriptions that are continually updated to catch the latest efforts of ad networks to circumvent these filters. I recommend the EasyPrivacy+EasyList combination filter subscription in the Miscellaneous section of the subscriptions page.

5. Cookie Culler
Cookie Culler is a handy extension to give quick access to the cookie manager in Firefox. It also provides the ability to protect certain cookies from deletion, but unfortunately, this behavior does not integrate well with Torbutton. Kory Kirk is working on addressing this for this Google Summer of Code project for 2009.

6. NoScript
Torbutton currently mitigates all known anonymity issues with Javascript. However, if you are concerned about Javascript exploits against your browser or against websites you are logged in to, you may want to use NoScript. It provides the ability to allow Javascript only for particular websites and also provides mechanisms to force HTTPS urls for sites with insecure cookies.
It can be difficult to configure such that the most sites will work properly though. In particular, you want to make sure you do not remove the Javascript whitelist for addons.mozilla.org, as extensions are downloaded via http and verified by javascript from the https page.

7. Request Policy
Request Policy is similar to NoScript in that it requires that you configure which sites are allowed to load content from other domains. It can be very difficult for novice users to configure properly, but it does provide a good deal of protection against ads, injected content, and cross-site request forgery attacks.