As everyone knows, it has long been possible to add a very fancy signature to your mail.app emails, using the excellent tutorial 
provided by Melvin Rivera.
It’s complicated, involving some digging around under the hood. It also requires a measure of HTML and CSS coding ability.
However, when the process is over the result is impressive (provided that you don’t hold a faith position on plain text in Internet communication):
Now a poster on macOSXHints has discovered an easier way. It is possible to add hyperlinks to your signatures in Mail’s Preferences without the pain involved in the first option.
All you need to do is type text into the signature field, highlight it and press Command-K (⌘-K). Up pops a dialog into which you place the URL, and you’re done. In a minute you have a hyperlinked signature, not as polished as Melvin’s, but easier on the eye than a sig full of long URLs:
Rob Griffiths comments that the tip didn’t work for him in 10.5, but it’s working fine for me in 10.6.1 (mail.app 4.1).
Of course, Command-K (⌘-K) also works in the body of any (rich text) email you are composing.
[Via macOSXHints ]
I’m not sure if it was available in Leopard, but in Snow Leopard you can add a link to your signatures by hitting the right mouse button and select Link -> Add Link if you forget the Command-K thing.
a) Copy Link in URL-Window (for ex. SAFARI)
b) Paste in Mail template.
finito.
The menubar remains active when editing signatures, so that’s why the Command-K shortcut was working. Choosing “Add Link…” from the Edit menu works equally well.
And since the menubar remains active, you can also use the full breadth of formatting options… choosing fonts, styles, sizes, alignment, etc.
Great tip! Thanks for the linkage.
@Melvin — No problems. Easier to knock out a link than a tutorial! And yours is a model of clarity.
I confirm :
This tip doesn’t work in Leopard Mac OS X 10.5.8 with Mail.app 3.6
It seems i need to upgrade…!
You do! And not just for this tip ;-)