This dll can be used to show a ComboBox with multiple CheckBoxes. We’ve fixed a few bugs and made it work with .NET 2.0.
http://www.codeproject.com/KB/combobox/extending_combobox/CheckBoxComboBox.zip

Licence:
The Code Project Open License (CPOL) 1.02
http://www.codeproject.com/info/cpol10.aspx