I am a Mac admin in my company with around 150 Macs. With off all these users, I find that I tend to repeat what I say in my emails several times a month. For example, we have an online help desk system that I like to write new users about, and attach a little "How-to" PDF in the email. I was doing this chore last night and I thought "there has to be an easier way of doing this... an automatic way..." The idea of using Mail.app's Signature feature hit me.
So here's what to do: write out the email and copy/paste it into the Signature window. Of course, you can write your whole email in there, but the window is small, and that's sort of a pain (but to each his/her own!). Go ahead and put an attachment into the email, at the bottom (this is covered in a previous hint) if you'd like. Then save the signature with a description that makes sense. Make sure you have don't have this signature as your automatic signature (that would be a little embarrassing) and that you have "Show signature menu on compose window" checked. Now the next time you are needing to spit out that e-mail, open a new message, pull down the signature menu, choose your letter, and wa-la -- there it is (attachments and all!)
[robg adds: When I read this submission, I emailed Brandon and asked him "Hmm, why don't you just use the Save as Draft feature to put the composed emails in your Drafts folder?" (which is what I do). He replied that, to him, anything in "Drafts" means "Working" or "Unfinished," not "Template." So if you have a similar mindset, use this workaround to create your templates as signatures.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20031227103919637