Three Ways to Do It
Sony's upcoming PlayStation Phone is the holy grail for mobile gamers: a high-end smartphone with a sliding GamePad. Unfortunately, it isn't out yet and hasn't even been officially confirmed. Plus, most of you are probably locked into contracts for your current smartphones. So how do you get a GamePad set-up going?
One option is the Game Gripper. This clever device requires no battery and doesn't use Bluetooth. Just snap it on over the keyboard and it functions instantly as a GamePad. Note that you may need to tweak your key bindings before the Game Gripper works perfectly.
You can buy the Game Gripper here for the Droid, LG Ally, Nokia N900, Samsung Moment, Devour and the BackFlip. It costs around $15 and comes with multiple color options.
Cheap As Hell:
Maybe you want to save a few bucks, maybe you just don't want to deal with the added bulk of a Game Gripper. Whatever the reason, the Tactile+Plus stick-on buttons will do you right. Just slap the plastic D-Pad on over the one in-game and put the plastic dots over any in-game button.
Since the buttons conduct to the display below, you don't lose out on any sensitivity but you do gain the ability to "play by feel". This is also a universal product, you can buy it now in or from Japan for about $8.
Complicated But Satisfying:
If neither of the above solutions fit your preferences, you may want to check out a nifty little product called the MSI BGP100 Bluetooth Gamepad. It isn't being produced anymore but you can still find stores with some in stock. They show up on Ebay from time-to-time at hugely inflated prices.
Despite the fact that this thing pre-dates Android (it works for Microsoft Smartphone 2003 OS) there is a very detailed guide that will help make everything work.