Most of the hammer multitools I know of look practical at first glance, but they are not. The handles are too short, which puts the hammer head too close to the user's hand. You might be able to pound a tack into a bulletin board with one of these tools, but not much else.
Much better are three of the tools mentioned by Lonediver above. These tools have fold-out extensions on the handles, which gives the hammer head some real momentum when you swing it.
The best hammer (in my opinion) is the Troika Obelix. Note that the folding handles in the photo pivot a full 180 degrees until they until they give the hammer head about a 9" long handle.
Another hammer tool from Troika is the Adlatus. It has a unique pair of tongs/cutters that close up to become a hammer. The tongs make the Adlatus a bit more versatile than the Obelix, but the head is lighter, making it less useful in the hammering role. The quality of both these German tools are absolutely top-notch.
Also mentioned above was the Galls Rescue Tool, which also makes use of the folding-handle-extensions technique. This hammer has some unique features for firemen and rescue personell. My only complaint is that the tool and sheath seem so big that I wouldn't be likely to carry it around with me.
Hope this helps!
~Bob