The best established attentional training game is called dual n-back, and you can download and use the software free. You can probably tell from the name alone that dual n-back isn't exactly the instantly addictive ‘Bejewelled Blitz', but it has its charms in an oddly meditative way. The game requires you to simultaneously remember letters (spoken aloud by the computer) and locations of squares on a tic-tac-toe style grid (presented visually, obviously). At the easiest difficulty level, you need only remember the letters or locations from the previous round. At harder difficulty levels, you need to remember letters or locations from longer ago (the round-before-last, and so on). Harder difficulty levels quickly become fiendishly difficult and demanding, but you can always ease off and practice at the easier levels.
On the PC, you can play the dual n-back game by downloading the open-source software Brain Workshop. On the iPad, you can download the IQ boost app (also free). Both of these versions of the game come with full instructions.
It should be noted that research in this area is at an early stage, and your results may vary. Anecdotal comments on various internet sites report a range of benefits, from significant to none at all. Still, it never hurts to try. A common theme is that it takes dedicated practice to achieve significant results.