The two basic good choices of 12 ga pump shotguns are the Remington 870 series and the Mossberg 500 series. Both come in every length, finish, shape, magazine tube length, etc that you could ever want and both are very modifiable as far as different barrels, tubes, stocks, grips, etc. Both are extremely well made (in the USA - if that matters to you) and dead reliable. They are both extremely easy to take apart, clean, switch barrels, and add accessories. Honestly, they are nearly the same shotgun and you can't go wrong with either. When I was looking for a shotgun, the major differences I found between the two models as far as basic shotgun design were:
a) safety position - mossberg puts theirs on the top where you can operate it with your thumb and remington puts theirs on the rear of the trigger guard behind the trigger
b) slide release - mossberg puts theirs on the trigger guard behind the trigger and remington puts theirs in front of the trigger guard
I'm an engineer who is picky about design and ease of use so these things matter to me alot. They don't matter to most people and or they just might have different preferences or reasoning. With that said, I find the mossberg design to be much better. I like being able to see the safety just by looking down, not having to twist the gun sideways, and look behind the trigger guard. On the 500 it is very easy to release and engage the safety without changing hand position. Same with the slide release - on the mossberg its in a more comfortable and natural place on the gun that allows you to use it without moving your hand as much.
I ended up going with a Mossberg 590, which is the semi-mil spec version of the 500. It has a heat shield, bayonet lug, 20" barrel, and a full length magazine tube (to the end of the barrel), so it'll hold 9 or 10 shells total. The 590A1 is the actual US mil-spec version which has a heavier barrel and I think a metal rather than plastic trigger group. IMO 20" is a good length for a home defense shotgun as its long enough to get a decent range, but short enough to wield in a house and get a decent spread with buckshot (BTW, 00 buckshot is awesome - its 8 .33 caliber balls in one shot!).
Other cool shotguns are Binelli tactical semi autos - like the SWAT guys use, which are very expensive, and the Russian made Saiga-12 which is basically a 12 gauge semi auto AK47.