Multi-select dropdown list in ASP.NET

You could use the System.Web.UI.WebControls.CheckBoxList control or use the System.Web.UI.WebControls.ListBox control with the SelectionMode property set to Multiple.

I've used the open source control at http://dropdowncheckboxes.codeplex.com/ and been very happy with it. My addition was to allow a list of checked files to use just file names instead of full paths if the 'selected' caption gets too long. My addition is called instead of UpdateSelection in your postback handler:

// Update the caption assuming that the items are files<br/> 
// If the caption is too long, eliminate paths from file names<br/> 
public void UpdateSelectionFiles(int maxChars) {
  StringBuilder full = new StringBuilder(); 
  StringBuilder shorter = new StringBuilder();
  foreach (ListItem item in Items) { 
    if (item.Selected) { 
      full.AppendFormat("{0}; ", item.Text);
      shorter.AppendFormat("{0}; ", new FileInfo(item.Text).Name);
    } 
  } 
  if (full.Length == 0) Texts.SelectBoxCaption = "Select...";
  else if (full.Length <= maxChars) Texts.SelectBoxCaption = full.ToString(); 
  else Texts.SelectBoxCaption = shorter.ToString();
} 

jQuery Dropdown Check List can be used to transform a regular multiple select html element into a dropdown checkbox list, it works on client so can be used with any server side technology:

_{(source: googlecode.com)}

Try this server control which inherits directly from CheckBoxList (free, open source): http://dropdowncheckboxes.codeplex.com/