Most fans of the Mozilla web browser suite are probably aware of Phoenix, the lightweight, cross-platform browser heavily based on Mozilla's code. Phoenix supports the same web standards as Mozilla, but it has a much smaller memory footprint due to it's stripped-down codebase, lack of integrated mail client, etc.
Well, here's more good news for speed-happy or low-end hardware users - Minotaur. Designed with the same goals as Phoenix, this will be a lightweight, cross-platform mail client based heavily on the Mozilla Mail code. You can read additional details in this usenet post, or on the project homepage.
I just want to also say that, although I personally prefer the standard Mozilla suite, this is a good thing. Older machines especially will benefit from this, as well as anyone that's put-off by Mozilla's sometimes long startup times (due to the fact that, unlike another web browser, it's not loaded as part of the OS).