I currently use these two most often:
http://converter.telerik.com/
http://www.carlosag.net/tools/codetranslator/
But have also had some success with these others:
http://converter.atomproject.net/
http://www.dotnetspider.com/convert/Csharp-To-Vb.aspx
http://www.developerfusion.com/tools/convert/csharp-to-vb/