The Internet Explorer WebControls is an awesome collection of ASP.NET server controls that gives you a solution for four UI elements: TreeView, Toolbar, TabStrip and MultiPage.
You can use these in your SharePoint web parts or user controls (loaded through SmartPart). However, there are a few things you need to be aware of in order to get it to work under SharePoint.
The treeview uses its own font and font size specified by the variables 'L_fontSize_Text' and 'L_fontName_Text' in the file 'treeview.htc' under '/webctrl_client/1_0/'. You can change this to suit your needs or preferably leave them as empty strings to use whatever is specified in the SharePoint stylesheets.