Twingly is a cool startup company providing a great combination of searching for blogs and linkback service for blog post comments of other sites' posts. Twingly is also working on a "visual blog search" tool called Blogoscope.
To use the linkback service, you just do the following;
1. Write a post in your blog and link to the article on another site.
2. Ping your blog at Twingly http://www.twingly.com/ping