Everything "Rhinoceros"
A lot of information about the play, including information about productions and the Theatre of the Absurd.
Rhinoceros: Dramatic Criticism
http://www.theatrehistory.com/misc/rhinoceros_002.html
This site approaches "Rhinoceros" from a dramatic theory point of view. It is not the most extensive, but it includes a nice variety of perspectives as well as quotations from critics.
Eugène Ionesco
http://www.kirjasto.sci.fi/ionesco.htm
This site provides basic information on the life of the playwright and his work.
Notable Quotes
http://www.notable-quotes.com/i/ionesco_eugene.html
This site has an interesting collection of quotations from Ionesco and other playwrights associated with the Theatre of the Absurd.
New York Times Theater Review
The 1961 review of the play in the New York Times.
Eugène Ionesco Timeline
http://www.theatredatabase.com/20th_century/eugene_ionesco_timeline.html
An interesting timeline of the major events in Ionesco's life. This site also has links to quotations and trivia.
Eugène Ionesco
http://www.fcps.k12.va.us/westspringfieldhs/academic/english/1project/99exist/ionesco.htm
This site puts Ionesco in the context of other Absurdist writers.