Place the primary keypad near the door leading to the garage or driveway. If you will have a "custom English" keypad, place it here since this is where you will most often interface with the system. Most families want a second keypad in the master bedroom. I like to place this one above the light switch, a few inches below eye level.
If there's a separate, formal entry, some folks like to add a keypad there as well. I don't often recommend this location though, since it is only used by company and you usually would have the alarm off when expecting guests. Also, most keypads don't exactly add to the appearance of a formal foyer.
If there is another place where you often go in and out while the system is armed, such as to walk the dog, consider installing a keypad there.
Napco still makes excellent products and they're (the products; not the manufacurer) still DIY friendly.
Everyone has listed the usual places. Here are some unusual places:
Beside or near the bed in the master bedroom. Suposedly to give the person easy access to the panic button and to be able to see which zone trips without having to go across the room to view the keypad by the door.
I also had one person leave it on a wire coming out of the wall by the nightstand in the master bedroom so the keypad could rest on a small stand, (customer provided) on the nightable.
In closets ..... to keep them out of view. Master bedroom and main entrance foyer.
Outside of master bedroom, in the hall. So the kids could disarm the system should they get up and want to go down stairs. Motion detectors on the first floor were active at night.
Inside a "safe room" on a seperate partition. The alarm panel and back up long range wireless radio was mounted in the same room with concealed conduit wiring to the telephone line. The guy had a collection of Uzi's and other firearms in there also.
Remember that most alpha numeric kepads are LCD. The viewing angle is important. Households with people who are short or with younger kids or people who have bifocals must be considered. Mounting it at your eye level isn't always the best place. Also consider that the entry sounder will be beeping at night and if mounted in the master bedroom or near sleeping areas, it could cause a problem if there are entries during someones sleep time. Some keypads will allow you to permanently silence the beep. Napco will allow you to silence the entry beep but if the chime feature is on, all the keypads will still beep even though you've chosen to silence the beep on the bedroom keypad.
Keypads are a relatively expensive item. Use them wisely.
You may want to consider where *not* to place a keypad. I always try to keep them as far away as possible to an internal speaker or siren. No sense in blowing out your eardrums trying to shut the alarm of after an accidental trip. Don't face them towards doors or windows where an intruder can see the status of your system. If you leave and forget to arm your system a burglar can get a free ride undetected.