According to RFC 1738, an exclamation mark is a valid character:
Thus, only alphanumerics, the special characters “$-_.+!*'(),”, and reserved characters used for their reserved purposes may be used unencoded within a URL.
Example valid URLs:
- http://www.cinemanow.com/Buy/Movies/1007,0,5,,1,2,0/1001,0,5,,1,2,335666/Airplane!.htm
- http://www.usda.gov/wps/portal/!ut/p/_s.7_0_A/7_0_2KD?contentidonly=true&contentid=2009/04/0101.xml
This is supported by all of your browsers and most CMSs because it is valid, at least according to the specs.