steganography[1], anonymous remailers[2] (particularly the Mixminion[3] type), the Paranoid Security Guide[4] (which was on HN today[5]), and a nice talk on privacy and anonymity options[6][7]
There were also a couple of lists of darknets on HN recently, but I can't find them right now.
[1] - https://en.wikipedia.org/wiki/Steganography
[2] - https://en.wikipedia.org/wiki/Anonymous_remailers
[3] - https://en.wikipedia.org/wiki/Mixminion
[4] - http://crunchbang.org/forums/viewtopic.php?id=24722
[5] - https://news.ycombinator.com/item?id=6040381
[6] - Part 1 - http://www.youtube.com/watch?v=HHoJ9pQ0cn8
[7] - Part 2 - http://www.youtube.com/watch?v=s9fByRmAHgU
Also, is there any serious steganography product that correctly hides plain text? (I know there are many proof-of-concepts and experiments and toys)
As we've seen recently, this kind of metadata is often just as revealing as the content.
Encryption for email is probably giving you a false sense of security. If you care about privacy, email feels inappropriate to me.
So then the question is, what is the alternative? Honest question—I feel kind of helpless about it.