I have created an AppleScript which will append a random quote to the bottom of your signature in Microsoft Entourage 2004. You can get the script from scriptbuilders.net (macosxhints mirror).
To use this script, you need to do a little configuration in Entourage. First create a signature called QuoteMeNowBase.
In this signature you can place any normal signature you like. Make sure you leave a blank line after the signature so the quotes don't run into your signature.
Then create an empty signature called QuoteMeNowSig. In your account settings, configure your signature to be the QuoteMeNowSig signature. Then create a schedule to rotate the quote. For instance, if you wanted to rotate your quote every minute, create a schedule that runs this script every minute. To rotate every hour or day, make that the frequency of the schedule.
Change the MyURL setting to one that you want. If you use my quote generator, you can discover a list of possible my= settings by visiting this URL. Click on one of the links, and you will see all the quotes for that category.
If you want to use the category, change your MyURL to look like this:
http://www.ryanhunt.info/fortune/getmyquote.php?my=somecat
Where somecat is a category from the list. To use a random quote from amongst all of the possible quotes, just use the base URL:
Mac OS X Hints
http://hints.macworld.com/article.php?story=20060811165325982