user5653: There are plenty of chess tactics trainers out there on the internet. What trainer works best for you? Why?

user3872: chesstempo.com. 
free
tactics gets harder as your tactics rating increases
can play standard or blitz tactics (rating adjustment dependent on time took or independent)

user4159: I'd agree that chesstempo.com seems to be one of the best, but I'd question how much use the basic (free) version is for improving your play. As the previous answer says, the problems get harder as you get them right which means you end up doing problems which are too hard for effective training. Also, you cannot choose themed problems.
The paid version, on the other hand, offers the possibility to build training sets based on quality, difficulty and theme. So, if you want a set of ~100 discovered attack problems which take you 30sec - 1min to solve you can easily request this, with the option of spaced repetition for failed problems.

user16817: Chessity has been my favorite site for a couple of years now. At first I sort of dabbled with the free materials they offered, mostly theme based problems. About a year ago I read a book called "Peak" which opened my eyes to the realities of effective training, and eventually I became a premium member at Chessity. There are so many ways to train on their site, and they WILL take you out of your comfort zone, which is one of the prerequisites of effective training. I'm glad to say hard work and their well thought out training programs are really paying off, it's feels great to demolish people who used to give me problems. I love the site, though I don't always want to do the work. As Jan Gustaffson said, "Chess is a constant struggle between my desire not to lose and my desire not to think".

5653: <p>There are plenty of chess tactics trainers out there on the internet. What trainer works best for you? Why?</p>
 3872: <p>chesstempo.com. 
free
tactics gets harder as your tactics rating increases
can play standard or blitz tactics (rating adjustment dependent on time took or independent)</p>
 4159: <p>I'd agree that chesstempo.com seems to be one of the best, but I'd question how much use the basic (free) version is for improving your play. As the previous answer says, the problems get harder as you get them right which means you end up doing problems which are too hard for effective training. Also, you cannot choose themed problems.</p>

<p>The paid version, on the other hand, offers the possibility to build training sets based on quality, difficulty and theme. So, if you want a set of ~100 discovered attack problems which take you 30sec - 1min to solve you can easily request this, with the option of spaced repetition for failed problems.</p>
 16817: <p>Chessity has been my favorite site for a couple of years now. At first I sort of dabbled with the free materials they offered, mostly theme based problems. About a year ago I read a book called "Peak" which opened my eyes to the realities of effective training, and eventually I became a premium member at Chessity. There are so many ways to train on their site, and they WILL take you out of your comfort zone, which is one of the prerequisites of effective training. I'm glad to say hard work and their well thought out training programs are really paying off, it's feels great to demolish people who used to give me problems. I love the site, though I don't always want to do the work. As Jan Gustaffson said, "Chess is a constant struggle between my desire not to lose and my desire not to think".</p>
