We’ve all heard of, played with or used the most popular Facebook applications -- Scrabulous, Owned! and (Lil) Green Patch come to mind. But what about the not-as-popular Facebook applications? With more than 27,000 apps to choose from, and the number growing daily, there are plenty of diamonds in the rough that may be unknown, but still stand out because they are particularly interesting, unique, viral or just darn fun.

Here are 10 Facebook applications that you probably have never heard of -- but should.

1) Traveler IQ Challenge -- This game tests players to find out the answer the question "How well do you know your world?" As a clock counts down, the game presents players with world cities, famous places and world capitals, at various skill levels. Players can only advance to the next level after reaching a high enough score. When players are done with the round, they get a ranking -- an overall ranking, and a ranking among their friends -- and they are given the opportunity to "challenge their friends" to compete. As well as being educational, this game is addictive. The interface is clean and easy-to-use, and the sense of urgency that the ticking clock provides is excellent.

2) iDescribe -- This application is based on the premise that people like to know what their friends think of them. The application makes it easy for friends to pick 5 words that best describe each other. The genius of this app lies in its inherent viral nature -- after signing up for iDescribe, there is no way to use it unless your friends are using it, too. In fact, when users sign up for the service and don’t immediately invite their friends, they are reminded: "You haven't invited any of your friends to iDescribe! So you may not be described back by as many people as you would if you invite some friends!"

3) Web Presence -- This application is really simple and useful. It allows users to feature their "social profile" by linking to their profile pages on various other social networking sites. This comes in handy for both business and personal use, as users can add a Web site or blog URL, or link to their profiles on LinkedIn, MySpace, Flickr, Twitter or any other site. There are some features that seem missing here -- it would be helpful if the app prompted users on the services that they could enter, it would be nice to be able to reorder the list, and the RSS integration that’s coming in the future will be great -- but this is still a simple, easy-to-use and helpful application that could quickly benefit every Facebook user.

4) Sketch Me -- This is another fun and very easy to use app. As soon as the application is installed, it immediately takes the user’s profile picture and creates a pencil sketch of the image. Once the user’s friends see their pencil sketched picture, they’ll want one of their own.

5) TWENTY20 Cricket -- First a huge disclaimer -- I don’t play cricket or know how to play cricket. But even so, this is a really cool game. The best thing about TWENTY20 Cricket is that when a user signs up, they build a team made up of themselves and 14 friends. Without the friends, there’s no game. And the application encourages continued use; the more that users "train" on playing, the more points they earn (and the more likely they are to win in head-to-head challenges).

6) Launch A Package -- This is the only corporate app that made the list, and it’s from FedEx. The tool lets you create a package to send to a friend, placing "gifts," photos or other attachments in a well-branded FedEx box. The most fun comes at the end of the process when the sender gets to slingshot the package to their friend -- who then has to download the application to open the package on their end.

7) Countdown -- The concept here is simple -- this application lets you set a date, and countdown to an event with a ticker that runs on a user’s profile. Since everyone is looking forward to something, this app has endless utility.

8) Who Has The Biggest Brain? -- Part game, part IQ test, this application is hugely addictive. Besides having a great name, the application tests users in four different categories -- analysis, calculation, memory and visual processing. At the end of the test, users are given a ranking. They are then compared to their friends who took the test, as well as the rest of the world and their hometown. This application continues to spread as people compete against their friends (and their own personal-best scores) to get a better ranking.

9) Where I’ve Been -- This is another travel-related application, but instead of being a game, it allows users to plot on a world map the places they’ve visited, lived and want to go. After filling out their personal profile, users can share/compare maps with their friends. The more places a person’s been, the more addictive this application becomes -- and the more that the creators of the app will reach their target audience. (Smart!)

10) Pixton Comics -- This may be the best "Facebook application" that isn’t really a Facebook application. Although you can reach the service through Facebook, users actually have to access the app on the Pixton Comics’ site. Once through the registration hoopla, users can create comics without having to draw -- either by choosing from templates that are available on the site, or by building their own comic from scratch. Once built, the comic can be embedded in any site -- including Facebook.

Melissa Chang is the founder of Pure Incubation, an Internet incubator based in the Boston area. She also has a blog about startups at 16th Letter.