Below are links to more on chess tactics; let me know of others and they will be added:
The wiki page on the subject is here.
Chess cafe tactics puzzles here.
Chess.com server: http://www.chess.com/tactics/
Tactics trainer: https://chess24.com/en/learn/tactics-trainer
Chess tempo: http://chesstempo.com/chess-tactics.html
http://www.ukchessblogger.com: an interesting chess blog. Check it out.
The Chessbase training site is here.
A new source for more tactical instruction: https://www.chessable.com/chess-tactics/
Michael Goeller has an extensive index of good online chess resources, along with some other interesting material devised by the author.
Mark Weeks's "About Chess" page is another great place to go to find learning materials -- his own, and links to those of others.
The University of Pittsburgh Chess Club Page has another impressive collection of links.
Rick Kennedy has an interesting new blog up on the Jerome Gambit.
Come back soon for more.