Although you have one main domain name, it's important to take a look at your domain strategy as a whole and check for things that may help -- or hurt -- your search engine rankings.
Redirect Domains to Main Web Site
If your main Web site is www.maindomain.com, then you'll want to 301 permanent redirect all the domain names you own that aren't in use to www.maindomain.com. There are several reasons for this, such as stopping mirror sites from appearing, making sure your main domain name gets credit for links to other domains you own, and making sure your main domain name gets the PageRank credit for links to other domain names you own.
This brings up another issue: the links pointing to other domain names. By doing some domain name research, you can find domain names that were previously on the same topic that might have traffic, backlinks, and PageRank. You can benefit by finding the right domains, buying the right domains, and redirecting them to your Web site with a 301 permanent redirect.
Choose the right domain name and you may benefit from better search engine rankings. If you were to find a domain name that was formerly on the same topic of your main Web site and you're able to buy that domain name and redirect it, it could mean additional traffic and additional sales.
Depending on how the search engines deal with that domain name, it could mean getting credit for additional backlinks and more Google PageRank to your site. And, if your domain is new to the Internet, there could be benefits to gaining some quick "authority" through domain purchasing/redirecting.
Buying domain names and redirecting them won't necessarily bring more traffic, backlink credit, or Google PageRank to your main Web site. There are many factors that might prevent this.
Whenever you buy a domain name, you don't know if traffic is already going to that domain name until you take ownership of it and point it to some place where you can look at the traffic (you could point it to your Web site, set up separate Web hosting for it, or use a domain parking service).
There are many factors that can influence whether you get backlink and PageRank credit, including whether the search engines give you that credit. Some search engines, such as Google, have been rumored to "zero out" PageRank and backlink credit when a domain name changes owner; they've also been rumored to "zero out" PageRank and backlink credit for other reasons, as well.
There are many checks you can perform before you buy a domain name, and that's probably best covered in a separate discussion. Those include going to the search engine and typing the domain name to see what comes up -- and searching for it in quotes to research it. You can also look at the domain name in the Wayback Machine to see its history.
There are several ways to find domain names, including searching at the expired domain name auctions such as Go Daddy's TDNAM.com auctions, Sedo auctions, and eBay. There are other domain auctions at NameJet, SnapNames, Pool.com, and TUCOWS Auctions. Most of these are covered all at FreshDrop.net, a domain research service that allows you to search those auctions for domain names. You can also sort the results using several helpful factors, including sorting by domain age, PageRank, and number of backlinks.
By doing some domain name research, you may be able to find domains you can buy that were on your site's topic that might bring some additional traffic to your site. And backlinks and PageRank might just come along with that on the side, as well.