Chess on the Go
I know of very few chess players who do not like puzzles. I can't think of a more enjoyable way to pass a few random minutes here or there than in sharpening my tactical sense trying to solve some poser. One problem, though, is that unless I plan way ahead and, additionally, have the means to tote some heavy tome, those random minutes that might be spent in such a pleasurable pursuit generally are spend twiddling my thumbs.
Kenn E. Thompson and Dwight K. Weaver have just produced a book, really an effort of love more than a commercial endeavor, called "Memphis Chess Club - A History of Problems." The book contains 500 problems and many fully games from the ultra-rich history of the MCC.
However, besides the physical book, there is an app for iPhones, iPads and iPods, containing the 500 problems only (which, according to the developer, Shawn E. Gano, who offers it at a phenomenally nominal cost of just $.99, "includes 500 interactive puzzles ranging from mate in 2 moves all the way to mate in 26 moves! The object is to finish each puzzle by forcing a checkmate in a given number of moves.").
So, for less than a dollar (or 5 for a penny), if you have the equipment, you can have 500 chess problems at your fingertips to help fill those random free minutes productively.