We come across many great sources of information while we plan our hikes. But these are the links we most consistently rely on for the most accurate information available. We also use physical maps, not referenced here, and, sometimes, GPS, while on the trail.
Pacific Crest Trail Association
Probably the most comprehensive source of information available. Kept up-to-date with links to Half-Mile maps, current trail conditions and a host of other links. Good place to start for anyone just starting out.
The best maps available. Period. Details include on-trail campsites, water sources, much more.
Download for Android and iPhone. Places you exactly on the map with markers for camp sites, includes water and elevation information. We now use both Guthook and Half-mile apps and find them consistent most usually. Guthook does include some additional sites and facts that Half-mile chooses to not include. Well worth the few bucks.
App for the iPhone (maybe available for other devices). Excellent app for PCT hikers. Places you via GPS on your exact location. If ever in question this app tells you definitively if you are on or off the trail. This app has saved us many miles of miss-turns.
Being Cancer Network
Excellent list of current blogs on cancer.