Basically, Hua Tuo allows you to recycle creatures from your graveyard by putting a target creature back on top of your deck. You must use this ability only on your turn and before attackers are declared. It’s still a great effect, as there are plenty of creatures that you’ll want to reuse during the course of a game, especially if you can grab that creature back without having to draw it for turn. The timing on this card is important, since it has to be on your turn, but there are ways to make it a valuable asset in some decks. No longer being a $60 card from Portal Three Kingdoms, Hua Tuo could now see far more Commander play. In fact, he's now become a bulk rare.
However, Hua Tuo's effect makes cards that manipulate the top card of your library much more powerful. Library of Leng, Scroll Rack, and Sensei's Divining Top are the first that come to mind. But there are some other crazy cards that benefit from top-deck manipulation as well, such as Elkin Bottle and Timesifter. The Enchantment Call of the Wild is especially good for putting creatures from the top of your library straight into play for 2GG.
This Hua Tuo Commander list from 2010 is a great list to glean ideas from, although plenty of cards that work well with the famous Chinese physician have been printed since then. Here's another list from 2013 based around a combo with Squirrel Nest. And if you're into Tiny Leaders, the Commander variant with only 3 CMC or less cards, here's a cool Hua Tuo Aggro list.
All told, Hua Tuo's cool ability may actually be pretty underrated, especially as a Commander. The Commander 2013 copies are probably never going to be worth much, so he's a fun cheap card that you can play with.