UPDATE [2009-04-01]: As you are possibly aware, Semisecure Login has not functioned with a stock WordPress install since before version 2.5. Moggy has created a new plugin (called Semisecure Login Reimagined) which is available for download here. You can check out his blog post about it for more information. This page is being kept merely for historical purposes (I’m a bit of a packrat).
The Semisecure Login plugin for WordPress increases the security of the login process by using client-side MD5 encryption on the password when a user logs in. JavaScript is required to enable encryption. When JavaScript is not available, the password is transmitted in plaintext (as normal), but authentication still completes in this case.
It is most useful for situations where SSL is not available, but the administrator wishes to have some additional security measures in place without sacrificing convenience.