Adding to the list, I use:
Rematch: Basically the same as Pet Battle Teams, an addon that allows you to save commonly used team configurations. It differs from PBT in that it has it's own little window instead of attaching to the existing UI and it has a TON more options. It allows you to create categories and save teams into different tabs to make things a little more organized. It also has a VERY advanced filtering/search system. You can search for pets that are strong/weak against certain things or pets of a certain level range in addition to the normal filters. And then perhaps the most awesome feature is that you can create a levelling queue for pets. You can add however many pets you like to the queue, arrange them how you'd like and then set out to level and once one pet hits level 25, if you have your team set up correctly, it will automatically advance to the next pet for you.
Downside is there is a lot of set up involved and it may be a little overwhelming at first, but if you have the time to really sit down and get to know the addon, it's AMAZING.
Pet Battle Keybinds: Very simple, lightweight addon that just adds a little quality of life features for those long battling sessions - it adds complete keyboard keybinds to all your pet battle functions. Swapping pets is as easy as hitting F1-F3 and cancelling a battle is a breeze by hitting 'F', no confirmation popups to click.