Say you wanted to find people who had an interest in asp.net development
The best way is to use the search form on the twitter.com home page:

Goto twitter.com

Type in the keyword phrase you are interested in 'asp.net development'.

A page will appear that has posts using that keyword phrase.

Click on the Twitter People and start following them, messaging them, etc.

Its a great way to get to network with like minded individuals in your chosen profession.

You can also use this technique to find potential customers or network on any topic business or otherwise.

Try it out and see who you end up meeting.