Just for the record to clear up some of the myths about Top Level Domains (TLDs)

Abbreviations:

.com (commercial)
.org (organization)
.net (network) <-- this often was internal use or sites such as webhosts

.gov (government)
.us (united states)
.edu (educational)

And the very new non-original TLD
.info
.mobi (wireless/mobile)
.travel
.name (individuals)

were the original TLDs and are the only ones standing for words

Originally .org, .edu .gov and .us were restricted and you had to prove you were said type of organization.

Nowadays only .edu .gov and .us have restrictions along with newest TlDs .travel and .mobi are restricted.

back in the early 1990s, it used to cost $50 per year with a MINIMUM 2 year registration to register a .com

$100 just to register a .com name

You could only do this through Network Solutions who had a monopoly and you had to be a techie to figure out how to do it all...(ie setting all dns records etc that happens automatically nowadays)

All the other extensions commonly used are actually International TLDs that stand for a country, but are commonly used otherwise.

Some countries have restrictions on who can register and own names.

A few popular examples include:

am - Armenia
be - Belgium
ws - Western Samoa
tv - Tuvalu
fm - federated states of MICRONESIA
eu - European Union

there are a ton more, but smart marketers and cash deprived countries decided to partner up so they could "brand" the extensions for other purposes.

.net used to be the number 2 pick in the early 90s, but soon people succumbed to popular marketing, and default started going to .com

9 out of 10 browsers will add both the "www."  and the ".com" if you type the name only and then press CTRL+ENTER.  There is no simple HotKeys for the "other" extensions.

IMO - due to the fact by default the average consumer assumes .com is the extension for a website - even to the point of telling people its .com when its not, ".com" will remain the most popular until a new naming convention comes up.

Right now -- .com and .org are king however, also coming into play now, due to prospectors and adsense hawkers, is .info

.gov and .edu are then next (when appropriate) in line of popularity of surfers.

Sadly, good old .net is last in most peoples mind when looking for a website.

The popularity of alternative extensions is due to domain prospectors who buy and sell names - with the hopes of turning a tidy profit

in 2006 Partners.com, FJ.com and TJ.com all sold for over $100,000.  That is JUST THE NAME, mind you, no site attached. 

SportTV.com and Announce.com were close with $99,000 and $90,000 respectively.

The fact that people will pay this kind of money for domain names, causes many to buy "high value" names of any extension.

High value names are names with 3 or less letters (you cannot register 2 letter domain names any longer), single dictionary words and common acronymns.

The top selling domain names, almost unwaveringly, all end in .com

In 2006, the highest selling non-.coms were Castle.net, Rebuild.org and Participant.org - -all going for between $5000 and $7400.  A nice sum, but not the 5 and 6 digits earned by .coms.

Only about 50 non-.coms even made it to the four figure level.

Just as popular as .coms are other International Domain names --due to the growing "e"conomy's and Internet Access of other nations - Primarily Europe and UK.

MusicDownloads.de sold for 10,000 Euros (about $13,000 USD)
Sponsoring.de sold for 6,901 Euros
StudentJob.co.uk sold for 5000 Euros

a bright note-- number 4 was Leads.us -- (GO USA!)  for $5000

Majority of the rest were Denmarks Domain, UK's .com and European Union's TLD

Ultimately, even words in other languages prefer .com - like CasaDelHabano.com (Cuban Cigar Store in Spanish) sold for $2800.

Remember, these are for domains only, no site, logo, corporation, customers, and usually no revenue.

.com is definitely king, and that's not going to change anytime soon.

*sales stats from the Domain Name Journal