One of the best ways that you can promote your website to Search Engines like Google, Bing and Yahoo is to be listed in relevant web directories.
Directories can be local, regional, national and international and it depends where you see your client market where you decide to get listed.
Directories also exist for specific groups, for example, do a search in Google for 'accountancy + directory' and you will find dozens of potential sources.
Free or Paid? This is a difficult one because it depends very much on the size of your market and the competition that exists. As a general rule we would recommend that you first take up any Free offerings/listings before paying for a listing and then only with those that appeared on the front page of any search results.
Whichever way you go try to keep a check on where new/potential clients have found you so that you can guage the effectiveness of your listings.
Where to start? To help get our clients promote their website Zenith Hosting supplies a list of relevant directories why not join us and get yours now?