You get more benefit from reading code if you study something very close to what you are working on yourself, something in the same domain, in the same framework perhaps, or at least in the same programming language, at best something you are deeply involved in currently

The Architecture of Open Source Applications

http://aosabook.org/en/index.html

AOSA gives a high level overview on many open source projects. It’s a good starting point to dive into the code of these projects.

Fabien Sanglard

http://fabiensanglard.net has some excellent code reviews on his website, particularly games.

You could read some of the code-bases he reviews, and then read his review. You’ll be able to compare and contrast your opinions with his.

Redis source

A cursory look into the source shows a very well documented code-base. Salvatore is an excellent C programmer and takes a lot of pain in writing good documentation.

Douglas Crockford’s JSON parser

https://github.com/douglascrockford/JSON-js/blob/master/json2.js

Worth a look because it is excellently commented and is easily understandable

Python Sudoku Solver by Peter Norvig

An elegant solution in one page of code.

Lua

http://www.lua.org/source/5.2/

The sources to Lua are pretty darn great. Here’s is Mike Pall’s ( the originator of LuaJIT ) guide to reading the Lua source code:

Postgres

https://github.com/postgres/postgres

PostgreSQL (any version, but latest is generally recommended) core. It is very well factored, and typically also very well commented. This community cares a great deal about code quality, because they are so clear on the relation between readability, diagnosability, and execution correctness

Slightly off topic, but Peter Seibel’s take on the idea of code reading groups, and the idea of code as literature, is interesting: http://www.gigamonkeys.com/code-reading/