The question comes up, um, periodically. Anyone remember the interrobang, pictured here? Invented in 1962, it was meant to communicate agitated incredulity. (It failed to catch on except with a few type designers.)
Now Mark Healy of Torque Market Intelligence in Toronto proposes a new mark to express a flatter affect. He calls it the pomma point: halfway between period and exclamation point, it suggests "mild excitement." You can view a short pomma point video here. (Hat tip to Grammar Girl.)
On the other extreme of the excitement spectrum is disco-punk band known as !!! (pronounced, according to the New York Times, "chk, chk, chk").
Which reminds me of the late, great Victor Borge (1909-2000), the witty Danish pianist who invented a system of "phonetic punctuation" in which every mark had a corresponding onomatopoeic sound. Watch Borge demonstrate the system to a very amused Dean Martin in a clip from (I'm guessing) the early 1970s.