There are many widgets that let you know who visited your blog recently. The readers are familiar with the "Recent Readers' widget provided by 'My blog log' and 'Blog catalog'.
There may be more but I know these two.
What if they even leave a message to you right on your blog so that you ca get back to them later? This can an effective way to keep in touch with your blog visitors and develop a business contact too.
Plugoo is offering a widget that enables the above facility.
Register for a free account and key in your IM id. Any IM id will do that includes Yahoo, MSN or Google. Then validate your email, come to 'Plugoo' and get the required HTML code to place it on your blog.