I remember once getting a lesson on the importance of commas:
Russian Tsars would issue verdicts by placing a single comma in pre-written papers that said "Death forbidden pardon."
Guilty: "Death, forbidden pardon."
Not-guilty: "Death forbidden, pardon."
And so, I was told, a single comma could save your life. There has been a resurgence in the life-saving virtue of the comma. Jest aside, a few years ago Canada saw a $2 million dollar comma dispute.
Occasionally, someone tries to invent a new punctuation mark, but they are rarely successful. A notable exception was the invention of the emoticon that now pervade informal written communication (of the typed, but not handwritten, variety). Emoticons were first proposed by Scott Fahlman on September 19th, 1982. The number and types of emoticons has grown to the point where they may be considered a form of micro-ASCII art.
Recently, there was an attempt to re-purpose the interrobang to represent a unit of levels of abstraction based on the very large, but mostly irrelevant, Cuil search engine.
Fun fact: The old Metaist logo used the because and therefore punctuation symbols from logic and math.
When have punctuation marks helped or harmed you?
David Malki ! and his use of an exclamation point as an honorific.
How to Use an Apostrophe at The Oatmeal for illustrated directions on the use of apostrophes.
Punctuation marks at Wikipedia for a long list of smybols.