Try chesstempo.
Untimed ranked Tactics

Thanks Scott, i know theres something like that on chesstempo but why using two sites? I would like to use one site for all of my chess needs - which is chess.com
For the whole packet and unlimited tactics on chesstempo you need a premium membership too and im not willing to pay for two memberships - one should be enough.
Hey there!
First of all, i would like to tell you guys that i enjoy chess.com very much, so thanks for that!
BUT what i would love to see in the future is the possibility to do untimed, ranked tactics. For now, your somewhat forced to rush the puzzles or at least there is some kind of pressure, which makes totally sense if your trying to improve in Blitz or something like that but not for someone like me, who likes to play games with 3days/turn.
So for someone like me its much more important to get the problem right, even if it takes ~15 mins. Unranked and untimed is an option but the possibility to see my improvements via ranking is much more rewarding and keeps up motivation. Im not a specialist about implementing things on a website but im sure it shouldnt be too much of a problem for you guys - so please - implement untimed ranked tactics!
Thanks for reading and keep up the good work!