This can be used or abused. I will post both ways and elaborate on how to go about this the ethical and business savvy way. I will make special note of how not to do this, so that you may avoid making a huge business building mistake (long term).
Domains that others have let expire that you can pick up for really decent prices. Most times as low as 7-10 bucks each!
Register Compass ($37/month)
dropday.com (limited free)
***I primarily use Register compass for researching domains and then one of the other freebies listed to confirm my results, as it really automates most of my searching for me.
So how it works:
A lot of people don't develop domains into sites, and then let them expire. Most of these have very little to zero traffic. Naturally, we do not care about these. We want expired domain traffic.
So we do searches on the expired domains that have good alexa ranking, doamin age, backlinks,PR (not as important),Dmoz or Yahoo directory listings etc. to determine which domains are winners.
One of the cool search functions I use frequesntly on RC is filtering by my keywords. SO now I not only find sites that have traffic still going to them, but they are related to my niche! (That means pretty targeted traffic!!!)
Just to make sure, when I have my list of winners, I verify they are indeed related enough to my niche. Then I register the domains in bulk and host them (setup a 301 redirect for all the backlinks to point to the root domain) and then redirect the main doamin (root) to my new site/new purchased site.
As an added bonus, with several domains pointing to your site tat have established rankings etc, it shows that your site is valuable to Google, so any work you do for SEO/organic traffic will be recognized faster and you will more credibility with your newer site. (It isn't perfect, but it works very nicely for getting traffic right away, and maintaining the folw of traffic tyo your site in the future)
When you sell your site/list it. You will need to make mention of your traffic and where it comes from. People will be impressed with your ingenuity (trust me). Especially if you do you homework on the domains and get good at picking winners (it ain't hard, think caveman)
If you fail to mention this, and/or do not sell the expired traffic domains with you listing...you are scamming. The buyer will inherit a site that does not have good traffic anymore! PLease use this responsibly and ethically. It makes good business sense to treat your buyers good. Because buyers are more times than not, repeat bnuyers if you treat them well.
Happy traffic snatching.