14ers.com: I probably don't need to even mention this since it is such a giant of a website on Colorado 14ers, but just in case you haven't seen this one yet, go to it now.
The American Southwest: Great place to get initial ideas on hiking areas throughout the southwest.
Joe and Frede's Hikes: My friends Joe and Frede hike a lot in northern Colorado and this is their informative site. Stats and maps galore.
Mike Weigand's Hiking Blog: Amazing trips around the country by friend and occasional hiking partner from Vermont.
Roger Ludwig's Hiking Blog: Nice trip reports from around the west.
Todd's Hiking Guide: Detailed trip reports for canyon country outings. For the Colorado Plateau region, this website is unsurpassed in usefulness.
Tom's Utah Canyoneering Guide: The name says it all. Nice place to daydream about spring canyon trips.
Wayne's Page: My uncle's hiking blog.
Ball, Stuart. 2000. The Hiker's Guide to O'ahu, Revised Edition. University of Hawaii Press, Honoulu, HI. 333pp. This book is so thick, I had a hard time believing there were that many places to hike on a small island like O'ahu, but this book turned out to be gold on my two trips there.
Hagen, Mary. 1994. Hiking Trails of Northern Colorado, 1st ed. Azure Publishing, Fort Collins, CO. 179pp. My very first guidebook, given as a Christmas present way back in 1995, my first year in Colorado. Boy have I used this book! I've hiked virtually every trail described, and the pages are falling apart, but what a nice book this has been. Highly recommended for northern Colorado hiking.
Kelsey, Michael. 1999. Canyon hiking guide to the Colorado Plateau, 4th ed. Kelsey Publishing, Provo, UT. 320pp. I hiked with Michael in Colorado several years back and he's incredibly knowledgable about the Colorado Plateau, having hiked all over it for decades. His outspoken political views have caught him some flak, but his line of guide books are second to none for sheer volume of information about hiking the deserts of Utah, Colorado and Arizona.
Lowe, Don & Roberta.
1973. 80 Northern Colorado Hiking Trails. Touchstone Press, Beaverton,
Oregon.176pp. Great selection of hikes for northern Colorado, wonderfully
described. This book is not only informative, but it has been very entertaining
for me to see how things have (or more often, have not) changed
in the last 35 years.
ArcGIS Online: Free seamless topographic map service of the entire US at multiple scales.
NOAA Sunrise / Sunset Calculator: Make sure you get to camp before the sun goes down.
UTM <-> Lat/Lon Converter
USGS Real-time Water Data: Is that river going to be fordable when you get there?
USGS Map Store: Every topo map ever made by USGS is here for free. The GeoPDF maps of 2011 are also here and are way cool.