Tsumego website with the nicest UI I’ve seen. Only ~600 problems available so far.
I don’t like tsumego in general. I feel bored and frustrated.
But I like very much this site. Very elegant and clean.
I worked my way up to 11k and now I begin to get stuck, but it was fun.
Today there are 692 problems on the site and they should increase daily (so they say).