I never know how my books are selling at my publishers' websites until I get a royalty statement from them. However, I can check my books' sales ranks at Barnes and Nobles as well as Fictionwise and I frequently do--though on Fictionwise it isn't easy because I must scroll through the lists. It is far easier to keep track of my sales on Amazon since there are several sites that will track my sales for me.
By far, my favorite tracking site is http://www.novelrank.com because that site will also track Kindle sales. It lists how many books were sold and when.
I also like Metric Junkie--mostly for their colorful graphs, though Kindle sales are not included. And for me, Kindle sales have been rather nice. :^)
For a simple listing of sales ranks, which is only for paper editions, I go to http://www.salesrankexpress.com/. This site will also list Amazon sales in other countries, too.
These tracking sites are helpful tools! Using them helps me to know if my promotional efforts have been effective.