BlankCanvas have a new release of their Gmail add-on: Gmail signatures.
I know Better Gmail 2 has an HTML signature option but I find it can be a bit ‘cranky’ – works some times, not others.
The BlankCanvas add-on works a treat (unless you’re using the Gmail Redesigned skin, in which case, again with the crankiness).
But if that doesn’t bother you, you can check it out here.