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).
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.