I have used Twellow.com. It is very good. I have also used search.twitter.com and then typed in terms. My best tool is hootsuite.com where I can create categories and lists to track people of a certain niche. If your business is local, you can also do an advanced search from search.twitter.com and enter in specific geographic areas, along with the term you are looking for.
Watch the trending topics list on the main twitter page too. This is a great way to join conversations if the topics are an area you are interested in. These are the active users and you can get some great conversations going to make Twitter more fun to use.