Websites: 

AcousticFriends.com - A networking site for acoustic artists and fans of acoustic music.
AmericanaMusicAssociation.org - The Americana Music Association's website.
AmericanaRoots.com - A great Americana website with news, podcasts, and its own radio station.
Americana-uk.com - UK site dedicated to Americana music in the UK and beyond.
Cheezeball.net - Quirky and funny website with its own unique take on Americana.
Euro Americana Chart - The Euro Americana Chart is compiled by DJs, journalists, retailers, promoters and other people who are interested in Americana music from all over Europe.
HickoryWind.org - An Americana blog with lots of great views and opinions on Americana. 

Americana Radio Programs and Podcasts Available Online: 

WoodSongs Old Time Radio Hour - A weekly Americana radio program hosted by Michael Johnathon and featuring a variety of folk, bluegrass and alternative country artists. Programs are available for streaming or download.  Video is also available.  This link is to the show's archive page.

WFPK's Live Lunch - Another great Americana radio program which airs weekly and is available for streaming online at your convenience.  The show is usually hosted by Scott Mullins and features a variety of folk, bluegrass and alternative country artists. Programs are only available for streaming. Again this link is to the show's archive page.

The Miller Tells Her Tale - Karen Miller's weekly podcast features new Americana music from a Scottish perspective.

The Medicine Show - Rob Ellen is another Scot with a love of Americana music. His Medicine Show features a variety of Americana artists and is available online. 

Magazines:

No Depression Magazine - The magazine which helped to launch the alt-country movement. 
Paste Magazine - An eclectic magazine covering a variety of musical artists with a focus on Americana.  Each issue contains a free CD of newly released music. 

Books:

Modern Twang: An Alternative Country Music Guide and Directory by David Goodman [439 pages; Dowling Press (October, 1999)] The ultimate guide to alternative country music.

South by Southwest: A Roadmap to Alternative Country by Brian Hinton [256 pages; Sanctuary Publishing, Ltd. (October, 2003)] An excellent look at the history and development of alternative country music.

Musichound Folk: The Essential Album Guide (Musichound Essential Album Guides) by Neal Walters (Editor), Brian Mansfield (Editor)  [1030 pages; Music Sales Corporation; Book & CD edition (October, 1998)] Though the title describes the book as a "folk" music guide, the book actually includes artists from the entire musical spectrum of Americana, including bluegrass, alternative country, blues and related genres.

All Music Guide to Country: The Definitive Guide to Country Music by Vladimir Bogdanov (Editor)  [975 pages; Backbeat Books; 2nd edition (November, 2003)] The definitive guide to country music from All Music.  This guide includes mainstream "pop country" as well as alternative country, bluegrass and folk.

No Depression: An Introduction to Alternative Country Music. Whatever That Is. by Grant Alden (Editor)  [256 pages; Dowling Press (October, 1998)] A collection of articles from the alt-country magazine "No Depression."

The Best of No Depression : Writing about American Music (Brad and Michele Moore Roots Music Series) by Grant Alden (Editor)  [300 pages; University of Texas Press (September 1, 2005)] The second collection of articles from "No Depression" magazine.