Topos theory reference suitable for undergraduates I am a third year undergraduate who has just learnt the rudimentals of category theory. 
My specialization is computer science, not mathematics. As part of my course work I want to write an essay on Topos theory. My professor says that it is possible to do so with my level (very little) of mathematical maturity, but I am not able to find any sources that treat this theory at anywhere near my level. Any suggestions?
 A: Have you seen the article An informal introduction to topos theory by Tom Leinster? 
http://arxiv.org/abs/1012.5647
The abstract says:
"This short expository text is for readers who are confident in basic category theory but know little or nothing about toposes. It is based on some impromptu talks given to a small group of category theorists."
A: Introduction to Higher Order Categorical Logic by Joachim Lambek and P. J. Scott. Try Google Books for this. The point of view is much more suitable for the functional programming aspects, even though the words "computer science" may never appear in the book. (Historically it is quite impossible to understand where toposes came from without sheaves, but technically starting from cartesian closed categories and adding bells and whistles is a shortcut.)
A: Along the lines of "Sheaves in Geometry in Logic", Ieke Moerdijk (co-author of that book) also wrote these lecture notes with Jaap van Oosten:
http://www.staff.science.uu.nl/~ooste110/syllabi/toposmoeder.pdf
I found them very good when I was first learning.
A: Elementary Categories, Elementary Toposes by Colin McLarty seems like it is what you want.
A: Many people would say this is a terrible suggestion, I think, but depending on your tastes and style, Peter Johnstone’s 1971 book “Topos Theory” might be good.
…true, it’s exceedingly dry, and has been described as “famously impenetrable”, and I certainly wouldn’t recommend it as an only text to try to learn about toposes from.  But I actually found it very helpful when I was first learning Topos Theory — first and foremost because it has really, really excellent exercises, with a big range of subjects and difficulties.  Secondarily, I also found that once I’d struggled tooth and nail to understand a construction elsewhere, I could come back to Johnstone and appreciate a really clear, neat, perfectly tuned presentation — albeit one I wouldn’t have been able to get anywhere with on its own.
A: Sheaves in Geometry and Logic, by MacLane and Moerdijk, is a beautifully written book on the subject.  It's one of the rare texts on such formal material that is fun to read, and is relatively easy from start to finish.
A: Ncat has a bunch of links: http://ncatlab.org/nlab/show/topos
In particular an outline of Johnstone's book is here: http://ncatlab.org/nlab/show/Elephant
I've been wanting to read it...
A: The internal language of the effective topos can be understood with requiring barely any technology and is lots of fun! For instance, Andrej Bauer observed that if you construct the effective topos using infinite-time Turing machines instead of ordinary Turing machines, then internal to the topos there is an injection $\mathbb{N}^\mathbb{N} \to \mathbb{N}$. See these slides for undergraduates.
A: I'm not sure how appropriate this question is for MO, but a clear candidate would be:
Topoi: The Categorial Approach to Logic, by Robert Goldblatt (another source).
It's free for download online, and it is pretty much perfect for what you're describing.
Another option could be Awodey's book.
