FireGPG is a neat little FireFox plugin that acts as a front-end for GPG and provides seamless integration with Gmail. Once installed and Gmail support is enabled (which is, by default), a series of signing/encryption related buttons will appear at the top of the Compose Mail page.
It also lets you easily encrypt or sign any selected text area on a web page.
It’s a very intuitive and effective plugin. I just wish I had stumbled upon this sooner.