Bang

Though its public persona is intrusive and abrasive, the humble exclamation mark is one of syntax's most important and yet delicate creations.

Common Uses of the Delicate yet Humble Exclamation Mark

Factorial operator
Exponential modality
Negation in C-like languages
Reference unboxing in SML '97
Typographic variant of '1' or 'l'
In pairs, list indexing in Haskell
Calligraphic error
Extremely makeshift up-arrow or down-arrow, depending how you look at it
Produces a vertical line on keyboards without a proper pipe key
In emacs, equivalent to M-x viet-encode-viqr-region under the correct bindings
With '#', interpreted executable escape in Unix
Comment character in Fortran '90
Under the Bourne shell, a varaible containing the process ID of the most recent background job
In perl, a variable containing a description of the most recent error condition
Solitaire, a pseudoword indicating an expression of surprise or alarm
Appears by convention in the names of mutating functions in Scheme
Frequently appears as a typo when inputting the string "1>1<1>1<1"