Be careful registering free domains. Usually all good and decent domain names are already registered. So there is probably a good reason why a domain has still been available.  If you find domains that can be interesting for a new technology or the top level domain (TLD) is new on the market, you might get a good domain. Otherwise it is not advisable to register free domains, at least at beginning of your domainer career.

One way to get decent domains is to catch dropped domains. My favourite catch services are Snapnames and Namejet. These services have many partner registrars that try to register dropped domains. If more than one person is interested in a dropped domain, an auction will be held. So if you want to get a good domain you sometimes have to pay quite a lot of money. From time to time you will certainly be able to get a bargain.

Another option to get lucrative domains is to participate on domain auctions. Usually domain auctions have quite a lot of competing domainers. So depending on the auction the prices can reach astonished levels. In spite of this you can get really great domains for a fair price that is often lower than the end user price. My favourite are the Sedo auctions. Here you can get good domains for reasonable prices. From time to time auctions are held on domainer events like T.R.A.F.F.I.C. Here you can find top generic domains, as well as average domains. It is important to mention that on every auction you can trap into a pitfall and bid far too much for domains, only because you do not want to lose the battle. This psychological effect can be avoided through setting a maximum price you are willing to pay.

Take a look on some forums like Namepros.com or DNForum.com to find some bargain domains. There a lot of rubbish, but if you search carefully you will able to make some good deals.  You also can find complete websites with steady traffic. To purchase a domain or a website on a forum I recommend you to use an escrow service like Escrow.com.

Of course you can just search on second markets like Sedo or BuyDomains.com for under evaluated domains. You will have some trouble to find a really good .com domain for a cheap price. This strategy I suggest you to use for not very strong TLDs like .eu, .us, .mobi or .asia.