A sight for sore eyes
A welcome sight; someone or something you are glad to see.
This phrase was first recorded by Jonathan Swift, in A complete collection of genteel and ingenious conversation, 1738:
"The Sight of you is good for sore Eyes."
The title of the work suggests that it was in use prior to his writing it down. The currently used version of the phrase was first recorded by William Hazlitt, in New Monthly Magazine, 1826:
"Garrick's name was proposed on condition he should act in tragedy and comedy... What a sight for sore eyes that would be!"
That's all pretty straightforward. The rise of the World Wide Web has given this phrase a new lease of life. 'Sight', 'site' and 'cite' form one of the small number of three-word homophone groups, i.e. words that sound the same but are spelled differently. Another three-word example is 'you', 'yew' and 'ewe'.
A quick scan of the Web courtesy of Google (Feb 2012) shows these hits:
"sight for sore eyes" - 1,550,000
"site for sore eyes" - 681,000
"cite for sore eyes" - 88,700
Top of the list for the latter two are web sites selling optical supplies and literary citations respectively, so they have some excuse. Many of the others are just mis-spellings.
See also 'four-word homonyms'.