There’s a whole list of websites that are trying to compile the working addons for the WoW Classic beta. The easiest one to parse through is Will It Classic. The site has a list of addons — including a search function — and has color-coded each one to signify whether or not it’s working on the beta client. Keep in mind that the client runs on the Legion API, so there are a few semi-current addons that work without any tweaking by their creators.
Blood: Unholy Command - Your Death Grip has two charges. Frost: Chill Streak - Deals up to 6% of the target's total health in Frost damage and reduces their movement speed by 70% for 4 sec. Chill Streak bounces up to 9 times between closest targets within 6 yards. Unholy: Necrotic Strike - A vicious strike that deals [ 30% of Attack Power ] Plague damage, and converts 1 Festering Wounds into a Necrotic Wound, absorbing up to 7% of the target's maximum health in healing recieved. Demon Hunter
Heirlooms are special pieces of armor that level up with your character, so they have stats that are always respectable for your character’s level (up to level 110). Heirlooms that fit the head, shoulder, chest, legs, and back armor slot offer an experience bonus. The total bonus is 45 percent if you have them all. It’s also possible to acquire a ring that adds another 5 percent, but you must win a fishing contest acquire it, and that can take some time.