I just discovered the Customize Google Firefox extension. It has a large number of user preferences which affect your use of Google services. Many of the preferences are privacy-related, including some anonimization features. It can also remove ads in some contexts, and add links to other search engines in some Google search results.
But this extension is interesting to me because it can force HTTPS traffic for Gmail and Google reader, which is especially beneficial for a laptop on a coffee shop wireless network, for example. A recent blog post on dmiessler.com makes a good argument (with packet-sniffing evidence) for encrypting your Gmail traffic. (One of the comments on that post is what directed me to the extension.)