I always recommend Maxim Blokh's biggest tactics set. It's now available in three formats that I know of. If I have time for it, I always work through a chunk of this before and during a tournament:

Physical book: Combinational Motifs
PC program (in the Peshka interface from ChessOK/Convekta): CT-Art 4 (or 3 or 5)
Android (and iPhone?) app: CT-Art 4.0

More training apps (not just on tactics) from ChessOK are being rolled out these days under the brand name Chess King. Worth keeping an eye on.

I also enjoyed working on the online chess servers Chess Tactics Server and Chess Tempo, though one should do a lot more of it than I did to have any effect.

Tactics books available as programs/apps that I've looked at and/or heard good things about:

The Steps Method: 5 steps which are supposed to take you from absolute beginner to around 1800. Based on a lot of repetition of typical themes

Chessimo: What I've seen of it reminds me of the Steps Method. There's both an online version and an iOS version (and possibly a PC program, but not sure about that). Android app supposed to be just around the corner.

Gambit Chess Studio: The Ultimate Chess Puzzle Book (Emms), How To Become A Deadly Chess Tactician (LeMoir), The Giant Chess Puzzle Book (Franco, forthcoming), John Nunn's Chess Puzzle Book (Nunn)

Chessbase: The series of tactics CDs by George Renko from the early 2000s, though I think they are hard to find these days

Forward Chess: Gormally's book on attacking play, Heisman: Looking For trouble (on spotting the opponent's threats), Chess Tactics From Scratch, Forcing Chess Moves ...

Everyman has tactics books too, like Aagaard: Excelling at Combinational Chess and the "Masters" series with tactics taken from a single player's games (one book each on Spielmann, Alekhine, Tal and Spassky).

I could go on and on really. All the ones I've mentioned are at least decent, but note they vary in difficulty, so you should check the recommended level before you buy/start something.

Hi all, I was wonder, what does everyone like to use to train there tatics on either a computer or Android device? I was looking to install something on my computer and phone to use when commuting to work or travel.

Oh for chess base (i like there format) are there any recommended tactics  cd's?

For andriod, I know there are different book apps like everyman, forward chess and gambit, do any of the apps do a good job (in terms of making easy to view and answer problems) and if so what books? No sure if that's a good option or just get a kindle book on tatics, but that could be a pain since no board and might be hard to look for the answer. Or maybe there is a better app? It has to be something I can use off line too.