1. PLANNED... There are a couple of wedding items that you really don't need to own, things you'll spend too much money on to wear for the day and never think about again. Passing them on to friends can become a fun tradition.
My college roommates & I all wore the same garter and then passed it along to the next bride to be.
Veils are another great option to share and I know of groups of friends who have done that as well.
Shoes! If you have a friend with the same sized feet, why not wear a pair that's already been broken in.
Also, think beyond what you wear as a bride. Borrowing from friends and family can be a great way to include them in the festivities and perhaps save you some money and add some more character to your wedding. Borrow the cool vintage car sitting in your groomsman's parent's garage (we did!) or "borrow" your aunt's amazing hand writing to write the seating chart.
Decorations are another GREAT thing to borrow/share with friends. Cake platters, vases, picture frames, etc. If you're planning a wedding, start looking at your recently married friends' Facebook photos, maybe there's something there you can use.
2. UNPLANNED... My favorite thing about "something borrowed" is it's way of turning a bride's moment of panic ("OH NO! I FORGOT MY ________") into a happy moment of tradition. When a bridesmaid quickly offers up her earrings/mascara/bra/deodorant/perfume it completely defuses the situation and immediately cuts out the stress.