Addons-Dps metter
Officially, ANet has said that they won’t take action against players who use DPS meters that only display the player’s contribution to DPS as well as party boons (i.e. stuff that you could theoretically figure out from watching the combat logs or UI). Two meters are popular, ARCdps and BGDM — you should google those to learn about how they work, their limitations, and risks.
ANet has infrequently mentioned certain types of programs that are never okay to use, such as using macros to automate more than one in-game "action, auto-clickers, and of course position hacking.
ANet has even more infrequently commented about overlays being probably okay to use, since they don’t interact with the game directly. Popular ones include GW2 Timer (shows nearby events and various relevant checklists for your achievements), GW2 TACO (shows you routes for farming, JPs, scavenger hunts), Immersive Combat Mode (what it sounds like), and Overwolf.
John Smith: “you should kill monsters, because killing monsters is awesome.”