HTTPS Everywhere is a free plug-in for Firefox and Chrome (beta only at the moment) that automatically uses an encrypted channel where ever possible
An extract taken from the download page:
Many sites on the web offer some limited support for encryption over HTTPS, but make it difficult to use. For instance, they may default to unencrypted HTTP, or fill encrypted pages with links that go back to the unencrypted site. The HTTPS Everywhere extension fixes these problems by rewriting all requests to these sites to HTTPS.
Download HTTPS Everywhere here