.AT

.at is the top-level-domain for Austria, the country of winter sports and the beautiful capitol of Vienna. A beloved country among tourists and Dutch emigrants, wich makes it an interesting country for developping online activities.
Interesting possibilities:
.at is highly populair in Austria, but also outside of that country, because of the similarities with the English preposition 'at'. An example of a url using that similarity is the travel website http://arrive.at. Apart from that, the .at domain has numerous possibilities, because a lot of words in various languages end with the letters 'at.' Think of magnific.at, appar.at, che.at or sal.at. Enough possibilities for creative registers!

.BZ

.bz is the top-level-domain for Belize, a country in Central America, located between Mexico, Guatamela and the Caribbean Sea. Until 1981 it was called the British Honduras and a British colony. The .bz domain is managed by nic.bz and can be registered without restrictions. It was being promoted in the 1990's as domain extension for businesses, until .biz arrived in 2001 and took over that market. Currently .bz is used for various purposes all around the world, such as websites related to the Italian city Bolzano and the Italian province Bolzano-Bozen.

.ORG

.org is the domain for non-profit organisations and open source projects. It is a domain that stands for reliability, for idealism, engagement and democracy. An .org extension communicates the message that a website is not made mostly for the benefits of the webhost, but for the user. So if you want to make a website that you want to come across as reliable and engaged, then .org is a good choice.
History:
.org was one of the six original extensions that were launched together in 1985. Apart from .org those were .com, .edu, .mil, .net, .com and .arpa. The idea was that each extension would serve a different target: .edu was meant for educational purposes and .gov for websites of governments. .org was the domain extensions for all the websites that did not fit in all the other categories. In actual practice it became the domain for non-profit organisations and other not-commercial enterprises. Despite this conscious choice for a target, registering a .org domain has never been restricted. Everyone who wants to register a .org domain, can do so. Because of this, there are also some commercial corporations that have a .org domain. This kind of use of the extension may be controversial, but it is not actively opposed to by the register. For a while, .org was also promoted as a domain for private websites and for informative websites, but for those purposes .name and .info were later introduced.
Current use:
Because of its non-commercial image, .org is popular with charity organisations and website that are related to the open source movement, where visitors and users can contribute themselves to the product or website that they're using. Examples of this are the website of the open source office software package http://www.openoffice.org and the online encyclopedia http://www.wikipedia.org. In 2009 it was announced that .org is the first generic top-level-domain that will start using DNSSEC, an initiative to fix the vulnerability for hackers of the normal DNS-zone. This only underlines the safe image of .org and makes it even more interesting. Another good argument to start considering ordering your own .org domain.

.COM

.com is worldwide the most used top-level-domain. It is so popular that for a lot of people it is synonymous with the internet itself. Of the 183 million domain names that were registered in June 2009, as much as 80 million were .com domain names. Having your own .com website therefore still leaves the most professional impression. Wich makes a .com website of your own essential for every enterprise aimed at the international market, wether it's a commercial or non-commercial enterprise, or even a hobby site.
History:
.com was one of the original six extensions that were launched together in 1985. Apart from .com, those were .edu, .gov, .mil, .net, .org and .arpa. The idea was that each extension would serve a different purpose and target group: .edu, for example, was meant for education sites, .gov for websites of governments and .com was meant as an extension for companies. Halfway through the 1990s the rules became less strict and VeriSign, the register of .com., decided to declare .com open for any kind of registration. Since then, the domain can be registered without restriction. In actual practice, the kinds of websites that the domain is being used for differ widely.
A household word:
During the explosive growth of online bussiness at the end of the 20th century, .com became such a household word that 'dot-com' became an entry in several English dictionaries. Also the word 'dot-com bubble' became generally accepted. This term indicates the period of 1997 to 2001, when numerous internet companies were founded and the growth of online commerce appeared to be unstoppable. Since the 'dot-com bubble' it is also common use for some (internet)companies to add '.com' to their company name. Examples of these are bol.com and Amazon.com. So it's safe to say that this domain extension has become a real cultural phenomenon.