Freelancing is the next best thing to being paid more for your full-time work, because professional work always pays more than unskilled. To find opportunities, let former colleagues or other personal connections that you’re available for freelance gigs. (Here are some ideas on how LinkedIn could be useful for that.) Or, post on marketplaces particular to your field. For instance, Mediabistro, a journalism site, allows freelancers to post profiles of their experience and services. Though these are more up to chance, designers can bid on jobs at or submit a design at Threadless, to see if it will be crowdfunded. Elance-Odesk also lists many freelance opportunities, and you can also post your own services on Fiverr, although some freelancers say these services create a race to the bottom on fees and so are not very lucrative. If you're new to freelancing, here's how to set your rates, and here's how to negotiate raises with clients.
These ways of making extra money cover a wide range in terms of compensation and prestige. Check out the options to see what could work for you, keeping tabs on a projected per-hour rate so you can see what would be worth your time. The list is loosely arranged by 1. jobs requiring more skill or expertise, 2. gigs needing less and 3. things you can sell. And if you think of any good options I missed, please let me know in the comments.

Mechanical Turk: Amazon's Mechanical Turk is a resource for doing human-intelligence tasks, or as the site commonly refers to them, HITs. You get paid a very small fee for any given HIT and you'll need a good deal of volume to make a substantial amount of money. But it is a resource you can use in your spare time to generate a small income online. 

A browser extension is a plug-in that extends the functionality of a web browser. Some extensions are authored using web technologies such as HTML, JavaScript, and CSS. Most modern web browsers have a whole slew of third-party extensions available for download. In recent years, there has been a constant rise in the number of malicious browser extensions flooding the web. Malicious browser extensions will often appear to be legitimate as they seem to originate from vendor websites and come with glowing customer reviews.[32] In the case of affiliate marketing, these malicious extensions are often used to redirect a user’s browser to send fake clicks to websites that are supposedly part of legitimate affiliate marketing programs. Typically, users are completely unaware this is happening other than their browser performance slowing down. Websites end up paying for fake traffic number, and users are unwitting participants in these ad schemes.
Français: gagner de l’argent (pour les enfants et les ados), Español: ganar dinero siendo niño, Português: Ganhar Dinheiro (para Crianças), Italiano: Guadagnare dei Soldi (per Ragazzini), Deutsch: Als Kind Geld verdienen, Русский: детям заработать деньги, Nederlands: Als kind geld verdienen, 中文: 赚取零花钱(针对小朋友), Čeština: Jak si vydělat peníze (pro děti), العربية: كسب الأطفال للأموال, ไทย: สร้างรายได้สำหรับเด็ก, Tiếng Việt: Kiếm tiền (dành cho trẻ em), Bahasa Indonesia: Menghasilkan Uang (untuk Remaja)
Wealthy Affiliate is also very strict on spam, which in the end is a GOOD thing, but I have to be careful sometimes about promoting my sites or referencing them. Sometimes I do have valuable information on my site that I think could help people during live chat sessions or Q&A’s, but I don’t share because I don’t want it to look like I’m spamming. They do allow people to promote their sites in certain areas, but in order to keep the community from filling up with self-promotion spam, there are times when it is best not to share certain information from outside sources, especially my own sites.
Blogging is something that requires patience, persistence and discipline. It may mean writing everyday for over a year before you really start to see any money from it. There are exceptions to the rule, but from my dealings with other bloggers, it seems to be pretty common to spend one or even two years building your blog, your brand and your authority, before making any serious amount of money.