I hate to contradict Shakespeare, who said "Neither a borrower nor a lender be." 
But I'm going to make the case for borrowing (and lending) stuff.
When times are tough and everyone's trying to save money, borrowing and lending things among friends is a great way to save money, if you're smart about it.
So how can you be smart about it? Glad you asked!
First, know what you will not, on policy, ever lend: more money than you're willing to lose (that should be about $5-$10 if you're a college student!), your car, or other high-ticket items, like your laptop, iPod, etc. Once you've made it your policy, it's easy to say, "Sorry, I don't lend out my (insert item here)."
Now, what items are good to lend/borrow? From friends, borrow things that you can easily trade back and forth, that neither of you wants to spend money on: movies , books, CDs, clothes — that kind of thing. When I was in college, I quadrupled my wardrobe of miniskirts, "MC Hammer pants" and boyfriend jackets simply by sharing with a few close friends.
Note I said "close friends." When you decide to start borrowing and lending stuff, keep your circle pretty tight. Don't lend your fave sweater to just anyone in the dorm, or you might not see it again. Or it might be a size too small when you do.
And don't be afraid to put your name on your movies and books to ensure that you get them back — even from well-intentioned friends. I borrow the latest bestsellers from my sisters all the time, and I'm glad they stick return address labels inside, so I always know where to return them.
So, while it makes sense to borrow small things, what about bigger things? I would stick to family for this kind of stuff — but it's definitely a good idea to try out an expensive item before you invest in it for yourself. Maybe before you buy a bike, try out the one with flat tires in your brother's garage, to see if you'll really use it. Not sure if you need a food processor? Borrow the one your mom hasn't touched in ages. Personally, I wanted to start sewing and considered buying a cheap machine. My sister piped up that she never uses hers, so I am currently borrowing it — saving myself the $75 I'd have spent on a basic model.
Once you get started, be a good borrower (Staci will laugh at this, as I once held a book of hers hostage for over a year). Return things promptly, in the condition you found them. And be willing to lend stuff back.
Do you think it's a good idea to borrow things to save money? Or did Shakespeare have it right?
There are some really cool apps for Mac's and PC's that allow to to catalogue things like books and movies, and one feature they often have is a check out feature. It can make things a lot easier to track of and on the one I use, you can even put a due dtae on it! They are also really cool to have if you are compulsive about the stuff you own. Like me!
My grandfather's motto on lending has always stuck with me - "Before you lend something, be fully prepared to never get it back.” Even the best of friends occasionally slip up and lose what you gave them, but it’s a waste to ruin a friendship over it. I prepare myself before-hand so that if I don’t get whatever I lent back, I won’t get mad. And when I do get things back, I’m always pleasantly surprised and pleased by the borrower. It also ensures that I don’t lend things I can’t live without!
And just for funsees – his other motto was “If you don’t want it repeated, don’t ever say it out loud,” and I think that’s just as pertinent now-a-days too. :)
@Matt - great idea -- especially for the super-organized among us (not me, necessarily!)
@Sarah -- your grandfather was a smart man!
For books, check out librarything.org. You can inventory your books online, find out what people with similar tastes have, read reviews, and (here's the relevant bit) tag books with custom tags, so you can note a book as "Lent out to Kelly April 21, 2009."
And yes, Sarah, I agree. When I lend money, particularly, I simply assume it's a gift. If it doesn't come back, no big deal because I didn't expect it to anyway: if it does, I'm pleasantly surprised, just like you.