TipHub

You have to start the search somewhere, I prefer a domain search site that will remember previous searches, find the most common extensions and is quick!

Note that just because you use a site for searching, it doesn't mean that you should use that same site for buying your domain! Domains generally range from USD$7 to USD$10.

Options

1. http://www.netsol.com - The best domain searching in my opinion. I'm a firefox user and for the longest time it didn't work with firefox, and it'll be one of the only applications I'll use IE to book my domains. Try it out it's great!
2. http://domains.aplus.net/ - This has a great suggestions list, and it's clear to see which ones has been taken.
3. http://www.godaddy.com - This is an average domain search site, but it's the most popular domain registry's out there! Check out this link for how popular they are! http://www.webhosting.info/registrars/top-registrars/global/.
4. http://www.domaintools.com/ - This is the swiss army knife of domain registrations, it'll even suggest domain squatters and how much it'll take to buy them. It has lookups of domains (whois), suggestions, allows you to monitor the domain, and lots more.
5. http://www.enom.com/ - An average searching site, but they're not the 2nd best registrar for no reason!.
6. http://www.namecheap.com/domain - Simple domain registration site.