Mossberg 500 Field/Security Combo. Comes with 2 barrels (18.5" smoothbore full choke, and 28" ribbed, ported accu-choke smoothbore) Get it with a wood stock--it's a little heavier, but in a home defense situation, you won't be lugging it through the woods for hours anyway, and if you run out of ammo and have to go "hand to hand", a wood buttstock is MUCH more effective than a polymer one.
Great for trap and skeet, good for hunting, and this particular Mossberg can often be had for well under $300. I got mine for under $280--out the door, sales tax included.
Mossberg pumps are the best-kept secret in the shotgun world. They are EVERY bit as reliable and rugged as an 870, at only about 85% the cost. Tons of aftermarket accessories for them are available, and they shoot very well, and are nicely balanced.
I put a sling and a brown Pachmayr Decelerator buttpad on mine and I can shoot clays with it all day long without getting beat up. It's a VERY natural-pointing shotgun, and the action is smooth and quick. It's a blast on the clays range, AND I trust it implicitly as a home-defense firearm, and have trained my other immediate family on it's use...
Plus if you ever are unfortunate to have to use one to defend your family in your house, they don't look all "tacticool" and it's going to be REALLY hard to convince a jury that you were some sort of mall-ninja rambo type when the prosecutor is holding up a plain-Jane wood-furnished pump gun....