Your friends can save you money. I'm not referring to Groupon or Living Social (you're already using those, right?) but rather creating a reciprocal relationship with your social circle that is financially beneficial to you both. Saving money can be a natural extension of friendship.
The classic example is borrowing a cup of sugar (or whatever) from your neighbor. It saves you from making a last minute trip to the store, which saves gas and possibly saves you from making impulse purchases.
Recently, when my internet went out, I borrowed my neighbors' WiFi. Going through that ordeal made me realize we could split the cost of internet with our neighbors and share WiFi. This would give us a 50% savings on cable internet. (We already have a wireless modem.) Chuk vetoed the idea for reasons I don't completely understand, but the next time our rate increases, I'm going to lobby for it again. For security reasons, you should only share with people you trust, but I totally trust our neighbors.
These same neighbors even have a key to our apartment. When we go on vacation, they watch our cat for us. In my area, having someone drop by once a day to take care of my pet would cost at least $20/day. (I live in an expensive area!) My neighbors have said we don't have to pay them, but we usually give them $5-$10 a day, which is still a big savings for us.
Our friendship has benefited our neighbors, too. When one of their cars died, we offered to let them borrow our car while theirs was being repaired. We only use a car about once as week for shopping, but they need two cars daily for work, so it was no trouble for us to lend them one.
When I was growing up, our next door neighbors' lawn mower died. Instead of buying a new one, they just borrowed ours' when they needed it. Sometimes, they would make their son mow our lawn too, so it was a win for everyone, except their son.
My final idea gets into murkier ethical waters, but you can also share warehouse club membership with friends and split the cost. Most of these clubs charge you about $50 a year for basic membership, which includes two membership cards. If you and your friend sign up for the membership at the same time, you can each get a card. My favorite local warehouse club is now frowning upon this, I think too many people were doing it, and it's not something I do, but I have brought my friends to the store when I'm shopping and we've just purchased our items in two transactions using only my card. As far as I understand, this is totally allowed. You could also just do one transaction and split the bulk items and the cost. This works especially well for people with limited storage space.
Do you have any more ideas for how you can save your friends money or how they can save you money?