Here is a list of essential tools to get you going:

• AjaxWhois is a convenient web2.0 domain search engine that executes multiple Top-Level Domain (TLD) queries on a root domain name. This can be a time-saver if you already have a specific root domain name in mind to purchase.
• dnScoop provides a variety of information about a domain, including whois, traffic, domain history and popularity, appraisal, pagerank check, inbound links and more.
• DigitalPoint is a set of webmaster tools geared more for websites and SEO’s, but which also provides some helpful tools such as a keyword recommendation tool which can help you come up with new domain ideas and word combinations.
• DomainState tools is a suite of browser-based domaining tools and links to trademark databases.
• Domain Tools has a suite of useful tools for domainers, including domain history, reverse IP (to see what other websites are hosted on an IP), DNS tools (whois, traceroute, etc.), domain monitor, and more.
• PagerankPredict is a very simple tool that gives you a predicted Google pagerank for a domain. This tool is handy if you plan to build out a domain with content. It is also free, but the site limits you to 5 queries per hour.
• SearchStatus is a robust SEO, SEM and domaining tool that works as a plugin for the Firefox browser. It will enable you to quickly get an overview of a potential purchase as well as a look and what backlinks are powering your competitors’ sites.