I use Act-on for tagging messages mostly — bills, client email, newsgroups, etc. But I also found it very useful for forwarding mail to my husband — some of our household bills and faxes come to me. Now I simply hit ctrl+P and it forwards the main to him with a cute little canned message from me, then deletes it from my inbox. Very handy.

to Vincent van Wylick: I use Mail-Tags in combination with Act-on. This allows me to use Act-on to assign key tags, then use smart folders — instead of actually moving the message out of my inbox (which is key with IMAPing).