WebSites you wouldn't want to be caught looking at by your boss, girlfriend, or grandmother.
Pornography sites are the canonical NotSafeForWork example.
You can use the 
http://nsfw.us/ link shortening service to keep people out of harm's way -- it asks for confirmation before redirecting.
2 pages link to NotSafeForWork:
lib/main.php:944: Notice: PageInfo: Cannot find action page