One of the easiest things you can do is take what you already have and sell it. A few resources to facilitate this task includes going to Plato's Closet, which is a store I have often utilized to get anywhere from $15-50 dollars by simply selling clothes. How awesome! Well, there is a catch of course - you need to have what they're looking for, so go to their website to see what styles they're currently aiming towards. They accept boy's items too, gentlemen - so don't be put off by the apparent female emphasis.
If you don't have a Plato's Closet in your area, just drop into a consignment shop and see if they will take some of your items off your hands. Some places operate differently than others, where one will pay you up front, another will not give you payment until the item sells. Either way, check it out, find out what their policies are - and don't be afraid to just ask. They probably won't take any of your latex clothing, but that nice dress you wore to Aunt Sally's wedding that one time might sell just fine! Another popular store is Once Upon A Child, and as the name insinuates - it is a store for children, so if you have any gently used childrens clothing or supplies, stop by and see what you can get for your goods. If you have used sporting goods, try Play It Again Sports. There are plenty of places looking to help you and help out by recycling unwanted or unneeded items you already have stuffing up your closet.
I've gone to pawn shops for everything from jewelry to electronics. If you have something you no longer use, but is of value (say a game console, digital camera, computer, piano keyboard, dvd's, cd's) they may take it off your hands. They will resell the item for about 50% less than it's original retail value, so expect to get about half of that. (i.e., Your camera was $200, they will sell it for $100, you get $50.) Call ahead or go in and find out if anything you have is something they might be interested in taking. Sometimes they won't be able to take what you have because they already have a lot of a particular item or haven't "moved" any yet, or the item won't sell well so it won't benefit them to pay you for it. Just pick through your possibilities and see if you have anything worth a shot.
A popular auction tool for people who make and sell their own items (clothing, jewelry, toys, etc) or want to sell items they already have is Ebay. Speaking of people who make their own stuff - is that you? Well, another great website to promote selling your own handmade stuff is Etsy.com. Sell homemade jewelry, perfume, handbags, note cards, whatever you make - sell it there. Check it out!
This one is my favorite. Every few months I go around the house and collect the coin jars (you have some, don't you?!) Scratch under the cushions, pick out pennies from the sock drawers (I don't know how they got there either...) and head over to my local Coinstar. Generally banks won't take your coins unless you've sorted, rolled and wrapped them, hence I have noticed Coinstar machines popping up inside the bank as well as grocery stores and other locations. Go to their website to find a location near you.
Take the needle and get some green. Well, while this might not appeal to everyone (and may not be available to everyone if a center is not nearby) but consider donating body fluids such as plasma if you're okay with a trip to the medical facility and down with serving a good cause. Be aware that the donation process can take awhile, anywhere from 45 minutes to an hour and a half. Give yourself plenty of time for the initial visit as you will have to go through a lot of paperwork and quite possibly a physical screen. For each donation of plasma you can earn anywhere from $20-30, and some people make a trip each week to donate. That's already $80-120 extra dollars in one month! If you're willing to give up some liquid gold, check out BloodBanker for facilities that pay for your contribution.
Along with selling items or donating precious body fluids you can also check out Craigslist for miscellaneous job postings, which can often include taking part in studies or doing odd jobs. Proceed with caution, but it's worth a scroll through. Still feeling helpless? Babysit, mow lawns, have a yard sale - there are a lot of ways to get money in a hurry, so whatever is fueling your need for some extra cash, try out some of these resources and get that money you need.