If you have a website or web application, you might want to try a website intercept product. Ethnio is the most well known of these, and UserTesting.com now does some intercepting for certain types of accounts.

The idea here is that you put a small bit of code onto your website and actually intercept users as they visit. A person comes to your site, and they get a small popup dialog asking if they'd be willing to join a user research session right then.

Pros:

• You get people who were already using your product.
• It requires very little scheduling.

Cons:

• It will interrupt the flow of people using your product, which may not be something you want to do.
• It requires you to have a web based product.