Pre-hike, there's a small handful of sites I like to check out in preparation. Some are more important than others depending on the season, such as trail conditions and wind chill forecasts in the winter and chances of thunder in the summer. I've listed them below but opted, with few exceptions based on extensive use, to leave out my favorite hiking blogs, as they're too numerous to list. I also have a handful of books, mostly by the AMC, with trail descriptions and suggestions.
NOAA - recreational report
National Weather Service - forecast
Mountain-Forecast - base and summit forecast
MWOBS - higher summits forecast
TrailsNH.com - there are others but this site collects recent trip reports from a comprehensive list of sources
Trail & route descriptions:
NH 4000 Footers
Northeast Hikes (we're on there!)
New England Hiking
A pretty extensive list of helpful and entertaining blogs here. In time, I'll try to add some more I enjoy and utilize.
Lists info (there are plenty more!):
52 With a View, map here