The Internet is full of "get rich quick" schemes. But most of them won't really make you rich and aren't very quick at all. However, if you need a little bit of cash and you need it now, there are a few legitimate ways you can make it online.
Sell your stuff. You can sell just about anything on Craigslist.com -- including some things you really shouldn't sell, but that's another story.
Look around your house and see what you can part with. Books, clothes, furniture... you name it. Make one listing for each item under its respective category in "for sale" or put them all under one listing and call it a virtual garage sale under the "garage sale" category.
Make sure you take pictures of everything and give as much detail as possible so people don't have to bug you with lots of questions.
If you live in a large city with an active Craigslist, expect to get bombarded with emails or phone calls, and expect to sell your stuff right away. If you set your prices just a bit lower than they are worth, you will probably sell them even faster.
You could have money in your hands within hours of making a listing.
Besides selling stuff, you could also sell your services - as a painter, babysitter, web designer, etc. Just be sure to stick with legal services, if you know what I mean.
If you aren't in a big hurry, you could also sell your stuff on eBay. This might be a better option for your more valuable items. But you will have to pay fees, and you'll have to pack everything up and ship it. Plus, it takes longer; the default length of time for an eBay auction is 7 days.
Raid your bookshelves. If you have tons of books that you don't read anymore and can bear to part with, you can sell them on TextbooksRUs.com and be paid within a few days.
All you have to do is enter the ISBN number to see how much the website is offering for each of your titles. If you choose to accept the price, you can add it to your cart. Once you have at least $15 worth of books, you pack them up and slap on a pre-paid FedEx label that you print out online. Take it to your local FedEx drop-off or schedule a pick-up and within a few days you will be be paid. Be sure to select "PayPal" as your payment method to get paid more quickly. Otherwise you'll have to wait a little longer for a check.
If you aren't in a hurry you could also sell your books on half.com or Amazon.com. You are likely to get better prices for them that way, but you'll also have to wait longer. In both cases you'll need to wait until someone comes along to buy your books. And even then you'll have to pay for shipping up front and wait to get paid by the sites.
Do piecemeal work for Amazon Mechanical Turk. You certainly won't get rich doing this, but if you need some money right away, and you don't have anything to sell, mTurk can put you to work, and you could have your money as soon as tomorrow.
If you aren't familiar with mTurk, it's basically a crowdsourcing marketplace for small tasks. Companies and individuals submit jobs that they need people to do - usually ones that only take a few seconds or minutes to complete. And then they pay anywhere from 1¢ and up for each task. Most of them are very simple and mindless. For example, you might be asked to look at a photo and determine whether the person in the photo is smiling or not - a simple task for a human but one that a computer would have a hard time with.
My strategy on mTurk is to look for the simple jobs that has a lot of HITs (Human Intelligence Tasks) that I can do over and over again as quickly as possible. Even then, your hourly wage is likely to be low. I average around $5/hour -- but it can vary wildly depending on the task. It's not much, but it's not bad for something you can do in your freetime from home. Some of the tasks are so mindless that you can easily do them while watching TV.
But be warned, mTurk is like the Wild West with little regulation. Amazon is just the middle man and does little to intervene in the many tiny transactions that go on there. So it is teaming with spammers and scammers who just want to get their hands on your personal information. Don't ever give out your email -- let alone your phone number, street address or social security number -- in the course of completing a task on mTurk.
Before you can get paid, you have to verify your bank account, which could take a few days. But once you do, you can cash out any time you reach at least $10. Although Amazon says that the transfer can take 5-7 days, my experience is that it usually takes only 1-2 days.
One caveat: Before you can get paid, each of the requesters (the people who "hire" you on mTurk) has to approve your work. This sometimes takes days.
There are a few other sites similar to mTurk -- including Microworkers and Shorttask -- but mTurk is the best of the bunch, in my opinion.
Write for the Internet. If you have even a little bit of writing experience and can provide samples of your work, you can write for Demand Studios and get paid within a few days. Typically, you will get paid $15 for each 400-word article you write, and payments are made twice a week, on Tuesdays and Fridays, via PayPal.
You do have to apply first, but approval is typically within a day or two, and then you can get right to work. Choose from the many thousands of titles available, write the article according to the site's guidelines and submit it to be edited. Within a day or two, either your article will be approved and your account credited, or it will be returned to you for a rewrite.
Keep in mind that your article does have to meet certain standards. If not, it will be rejected and you won't get paid. Too many rejections will get you booted altogether.
There are other sites that you can write for as well -- including eHow -- but none, as far I know, pay as quickly as Demand Studios.