Websites you should check out
My own trip planner
I've compiled a list of trips, primarily for the Sierra Nevada, from books and magazines as well as general internet sleuthing. Includes details like if a dog or campfire is allowed. Always a work in progress.
Weather.gov
A great site to get a weather forcast for specific point and elevation (as opposed to a nearby city).
NOAA Snow Depth
This site isn't entirely accurate (usually underestimates) but it's good for getting idea of the snow depth so you can plan accordingly. Another site that uses the same data and adds the PCT trail is PCTA.org.
Wilderness.net
Shows the boundries of all wilderness areas (national parks and national forests) as well as the trails if you zoom in far enough.
Natural Atlas
Very detailed trail maps and aggregates trips from various sources.
Yosemite Hikes
Has all the hikes in yosemite listed with ratings and pictures. Everything you need to know all in one spot.
Backpack the Sierra
Variety of Sierra information including early season trips and general tips.
Outdoors Generation
Beginner's guide to backpacking.