My GF and I like Fruits Basket
a lot. Just the other day we started discussing how "poly" it seemed. The anime is only 13 episodes long, while the manga is much, much longer and is still being added to, I think. I've only seen the series, but based on those episodes, it's a pretty good example of balancing multiple friendships/relationships (and working on understanding yourself and others). I was certainly cheering the V relationship they refused to resolve in the classic romantic comedy love triangle ways.