When my horse has big tangles I love Cowboy Magic detangler. It works so well and I can get even a dreadlock out pretty quickly. Otherwise I use Fructis Sleek and Shine leave-in conditioner to keep the mane and tail healthy. My horse's mane and tail have grown A LOT with the help of that stuff.
My favorite grooming tool ever is this Grooma Rake: Grooma Rake - Grooming Tools from SmartPak Equine
My horse has a two-foot long mane plus a tail that now would be dragging the ground if I didn't trim all the time. This tool makes combing all of her hair out take a less than a third of the time than if I use my fingers or a mane comb. It doesn't tear the hair out and it's pretty sturdy, too. It's lasted me a couple years so far with no signs of wearing out. I also use it on my bunny's sometimes-matted fur. Love it!