I've seen hints that media files can give you codes but I haven't been able top figure or any from the media I have. How does it work?
Right now all working passcodes I know are the one being published by Niatic Labs in twitter.
Here is the document which contains all known passcodes. All of passcodes should still be applicable and can be used by each player only one.
I recommend you use http://ingress.com/intel to apply them.
Would be extremely interesting to try to hack documents you get for codes, but so far I had no success.
Update: some very clever guys from Berlin resistance have found a way how to hack these document, this is the result:
11/19/2012: Sensitives (SOLVED) http://commondatastorage.googleapis.com/pac/IB/00/item/Sensitives1.png
Binary code is encoded in the right margin of the document.
Replace... Dots with 0 Dash with 1 Equal sign with 11
...to get: 0110000101100110011100100110000101101101001100010011001101110100
Convert binary to ASCII to get: afram13t an #Ingress passcode
An uncensored version of this document was obtained from portal hacking by Ingress >players. The faint text in the enhanced image (attached in this album) is lifted from >page 2 of the doc, which was also obtained by hacking.
Page 2 contains an ADA image hidden in the Niantic logo.
Apparently you need to find the code somewhere in the media.
For example, this page describes how the passcode ezekiel27w was recovered from a media by converting a string of digits in the header from decimal to ASCII.
(This means that those with full time jobs are unlikely to benefit from many passcodes)
(obsolete) The Investigation Board released new media containing clues which can be decoded to reveal passcodes, but it's ancient now. So no luck there. But you can still see the way people decoded them in these docs.
Follow the Ingress Google+ page, there are often clues in the pictures
Watch Ingress Reports, there may be clues there too
Get in touch with local faction members - they are what the game is made for, and often your best source of support
Some bottles of HINT water have codes under the cap!
These are generally discouraged as it takes the fun out of decoding.