The Official Wiki - One of my favorite wikis in all of gaming. Big shout-out to everyone that has contributed to it during the years. You can even type /wiki in-game to instantly open it up with your topic of choice. (ex. /wiki boss timers or /wiki *shift-click item*)
GW2Timer - Some events/world bosses function on a timer. This site allows you to keep track of what world events are going on and has a useful “Subscribe” function that will give you audio notifications when the event you’re interested in will be starting (as long as you have the site open). In addition, this is the easiest way to find jumping puzzles (complete with wiki links) and find points of interest that you’re missing. *Recommendation* Click on Map, right-click on any area of the map, click on Zone and then Toggle World Completion.
Upon zooming out, the world map will look more like the below screenshot:
MetaBattle - All-around build database. Includes builds for all game modes. Leveling builds contain detailed explanations.
Masel - Masel creates fantastic builds that are often simplified versions of "meta" builds
Accessibility Wars - A site featuring builds that are accessible and aimed towards players that want to use something that is a little less physically straining to play, but is still effective
Discretize - Fractal builds/guides. Not meant for open-world use.
Snowcrows - Raid builds/guides. Now features builds for open-world use!
GW2Skills - Build editor
GW2 Crafts - Up-to-date crafting guides
GW2 Efficiency - A suite of useful tools including a crafting calculator, inventory search/viewer, and so much more!
GuildJen - Achievement/collection guides in addition to guest contributors for other guides such as PvP and WvW.
Guild Wars 2 Music Box - Best way to compose songs/load up existing songs for in-game instruments. A little out of date these days.
[fast] Farming Community - Resource to maximize gold farming activities in-game
Peu Research Center - Another resource to help you make gold through open world activities.