In descending order of desirability, something to consider before you go ahead and register the domain for your next killer site!

.com, .com.au
The granddaddy of the internet boom, .com is the TLD to have – assuming it’s not already been taken, of course!
.net, .org, .net.au
The two other non-country specific TLDs are always nicec to get, if you can – they lack the familiarity of a .com, and for some sites that simply won’t do, but for certain applications or audiences a good, short & easy to remember .net or .org address can be very effective.
.co.uk
If you’re targeting a UK (or any other relevant country, for that matter), then the country specific commercial domain is another good choice – good enough for Modern Life, in fact – and in the UK at least, the .co.uk domain has a similar level of saturation in the commercial sector as the global .com.
.org.uk
Although relatively new, the .org.uk domain is quite desirable – not as much as .co.uk, nor .org, but compared to the rest of the newer TLDs- this one has potential.
.us
Made popular by del.icio.us, the .us TLD is great for those targeting a primarily US-based market, but lacks the appeal of a good .com.
.eu
Although not in widespread use, the newer .eu domain is limited in usefulness to countries within Europe – but if your business trades exclusively in this region, the .eu TLD is short and easy to remember – and it’s a lot easier to get hold of a good .eu domain than a half-decent .com!
.biz, .info
Unfortunately, early adoption by spammers and other less reputable sites have sullied the .biz and .info domains somewhat. I’m surprised no-one has registered “made-for-adsense.info” – perhaps that’s too obvious?
.me.uk
As with the more global .name (see below), .me.uk is intended for individuals, and for that purpose it’s ideal – unfortunately, it’s suitable for only that purpose.
.name
Intended for individual use, .name hasn’t really caught on, and so lacks the awareness of other TLDs. It doesn’t exactly roll off the tongue, either.
all others
There are hundreds of other country-specific and industry-specific domains available, but most lack the recognition required outside of the country in question, so for a global site it’s safer to stick to a generic TLD. Some country codes have gained limited acceptance in niche areas, such as the Federated States of Micronesia (.fm) for music sites, Tuvalu (.tv) for TV sites, and to a less useful extent, the Cocos (Keeling) islands (.cc). Unless you’re desperately short of ideas, these should be avoided as they can cause confusion with your visitors.To recap, the domain you choose has more ramifications than just search engine performance – in theory, SEs view all TLDs equally. The problem with strange TLDs is that:
  1. They can confuse visitors
  2. They’re more difficult to remember than the ubiquitous .com
  3. They can make you look less reputable than you actually are.

All of which certainly can affect your long-term site rankings. Stick to the good domains, if they haven’t all been taken.