Here are all the General Top Level Domains (gTLDs). As you can see a lot of them are very rarely used, mostly due to restrictions by the registry. The intent at ICANN was to have a TLD for each industry, taking the pressure off .com.
The efforts by .pro to become mainstream (and therefore make a lot of money) show the flaws in that idea. I suspect that the trend to open up extensions to all comers will continue, .org pointed the way. Watch .cat, in that regard. I have no idea if the people who proposed that extension had hidden plans, but the potential of a lot of money from cat lovers could eventually sway that registry.
.aero = aviation ...
.arpa = Internet infrastructure ... This one is for the machines themselves --- who knows what they do when we aren't looking? A photo spread of the server of the month?
.asia = Asian business and individuals. Expected to open soon.
.biz = business ...
.cat = Catalan community ... (An ethnic group in South West Europe)
.com = Open, generally commercial ...
.coop = cooperatives ...
.edu = postsecondary education, mostly US ...
.gov = United States Government ...
.info = Open, Intended to be informational sites ...
.int = organizations established by treaties ... (a relic of the early days, I doubt there are many sites left)
.jobs = employment ...
.mil = United States Military ...
.mobi = mobile electronics optimized ...
.museum = museums ...
.name = individuals ...
.net = Open, intended to be Internet related ...
.org = Open, intended for non-profits ...
.pro = professionals ...
.tel = Telephone ... Approved, not yet open.
.travel = travel industry ...
There is a page explaining each on my website, the first site listed in my sig --->