There are lots of controls (even free controls) that are available in market that can be used for Left Navigation.
Depending on the requirement and type of features expected, you can decide what to pick. It can be server control, javascript, DHTML or jQuery control.
For example, you can look at the free ajax controls provided by microsoft that has rich UI interface and good client interaction:
http://www.asp.net/AJAX/AjaxControlToolkit/Samples/Accordion/Accordion.aspx[
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-Sandeep