<p>It's great to find all those little snippets of code on the Web for your current needs, but is there anything better than getting whole application source code at once and reading it like a book? 
There's no better way to learn positive and negative aspects of various architecture solutions.</p>

<p>That's why I was trying to find some known/big websites with its source code published, but haven't found anything more than I knew already ( <a href="http://code.reddit.com/browser" rel="nofollow noreferrer">http://code.reddit.com/browser</a> )</p>

<p>Do you know anything more? </p>

<p>PS. Just being curious - have you heard about any unofficial big web site source code leak?</p>

Wikipedia's source code (MediaWiki) can be found at
http://www.mediawiki.org/
The most relevant parts are
"phase3"
(the core code; this strange name is because it was rewritten a couple of times)
extensions
(Wikipedia uses several of them; which ones can be seen at
Special:Version
)
Additionally, here is a very detailed explanation of Wikipedia's whole architecture:
Wikipedia: Site internals, conﬁguration, code examples and management issues
.