The Excel Addict newsletter by Francis Hayes - Great Excel tips. We learn something all the time from the newsletter. And the tips are sent out weekly.
Windows Secrets newsletter - Focus is on Windows and PC issues. Well worth reading. And also weekly! It is our favorite non-Excel newsletter. We normally drop everything and read it when it pops into our inbox.
Allen Wyatt's Excel Tips newsletter - Allen does an excellent job of helping one learn useful features in Excel. The sign-up link is on the right side. You may have to scroll down a little to see it! We have been subscribing to it for more years than we want to mention :).