Gear Stores:
REI: http://www.rei.com/
REI is by far my absolute favorite store to shop at for all my outdoor needs, especially backpacking. Their 100% satisfaction guarantee is amazing! They let you return anything if you have had any problems with it at all no questions asked and are super accommodating about everything. Their gear and clothing selection is phenomenal and their prices are great as well. Plus, if you are a member, at the end of the year you get 10% back on virtually all the items you purchased that year, which in my case, is often substantial.
Whittaker Mountaineering: http://www.whittakermountaineering.com/
An online store aimed more directly at mountaineering as opposed to REI so sometimes their selection is more diverse and their prices seem to be comparable.
Maps:
US Geological Survey: http://www.usgs.gov/
The USGS website is a great source for topographic maps of the U.S. They have a lot for sale but the best part is their tool called the Map Locator which lets you locate your area of interest on Google Maps and then overlays a grid on top showing where all of their 7.5 minute and 30 x 60 minute maps are located. You can then click on each grid and download a pdf file containing that topo map FOR FREE! Although this means you have to print the map out yourself, it allows you to get maps of exactly where you are going and even edit them on your computer ahead of time so you don't have to carry around a bulky map from the store that often covers a much larger than necessary area. REI also sells waterproof map paper in the legal size so that you can easily print out these USGS maps and take them with you on your trips without fear of them getting torn up or smeared.
Trip Planning Information:
US Forest Service: http://www.fs.fed.us/
The USFS website has a wealth of information if your trip is going to take you through one of the states many national forests. Their website can be a little confusing to navigate at times but if you are persistent you can find the information that you need. Often your best bet is to use the site to find the phone number of the ranger station in the are you are going to be in and talk to a ranger directly if you are having difficulty finding the necessary information online like I often do.
Slackpacker.com: http://www.slackpacker.com/
This website is a great resource for info about hiking/backpacking trails, trip reports, photos, general info, etc. They provide links to lots of other hiking-related websites and to a large number of personal web pages like mine!
Guiding Services:
California Alpine Guides: http://www.californiaalpineguides.com/
Guiding service I used for my 3 day Intro to Winter Mountaineering trip to the summit of Mt. Lassen. Our guide was excellent and very knowledgeable. Their online reservation system is quick and painless and respond to personal e-mails in a timely fashion. Their trips are a bit pricey but this one was definitely worth it and I hope to use their services again in the future....they guide all over the US and internationally so there are plenty of other trip opportunities.
Shasta Mountain Guides: http://www.shastaguides.com/
Guiding service that ran the Mountaineering Guide School I participated in with UC Davis' Outdoor Adventures Program.
Miscellaneous:
Backpacker Magazine: http://www.backpacker.com/
I just started subscribing to this magazine. I loved the first issue and they have a lot of great info, tips, maps, etc, on their website.
Outside Magazine: http://www.outsideonline.com/
I just started subscribing to this magazine as well. Its more focused on all outdoor related activities as opposed to Backpacker Magazine and tends to focus a little more on people.
Outdoorzy.com: http://www.outdoorzy.com/
A social networking site aimed at outdoor enthusiasts. Allows users to create a profile, upload a few photos, share interests, rate and discuss gear, submit trip reports..etc. I'm a new member so I can't comment on it too much more than that but so far it looks pretty cool. Overall my experience during the first week has been great but unfortunately there aren't a lot of active users yet so sign up!
Specific Location Information:
Desolation Wilderness
http://www.fs.fed.us/r5/eldorado/
The Lost Coast
http://hike.mountainzone.com/2003/lostcoast/html/index.html
http://infolab.stanford.edu/~crespo/lostcoast/
The Trinity Alps
http://www.fs.fed.us/r5/shastatrinity/
http://www.fs.fed.us/r5/shastatrinity/about/units/trmu/taw.pdf
Mt. Shasta
http://www.climbingmtshasta.org/pdf/ms-climb.pdf
http://www.shastaavalanche.org/
http://www.swsmtns.com/equiplistpdfs/ShastaEquiplist08.pdf
Mt. Lassen
http://www.nps.gov/lavo/
Tahoe National Forest
http://www.fs.fed.us/r5/tahoe/
http://gorp.away.com/gorp/resource/us_national_forest/ca/hik_taho.htm
High Sierra Wilderness Areas
http://www.fs.fed.us/r5/inyo/recreation/wild/howto.shtml#vclocations
http://www.fs.fed.us/r5/sierra/publications/pdfs/rogs/johnmuir.pdf
http://www.fs.fed.us/r5/sierra/publications/pdfs/rogs/ansel-adams2006.pdf