Kiershar has constructed the most ingenious monster trap I have ever seen. Unlike other traps, this one does not require a dungeon, and it will spawn every kind of mob, giving you all the arrows, gun powder, web, and feathers you will ever need. The design is perhaps slightly too large to fit inside your existing base, but on the upside, you'll never have to craft a single arrow ever again!
Personally, it isn't about the items. After the number times I have been slaughtered by mobs deep in my mines, I just like watching them die. Is that shallow of me?
If you want to see the trap in action for yourself, you can download the map here.