Another thing that took me far too long to find today... there's a lot of links to Steve Dunn's tool, including the CodePlex version, which hasn't been updated since 2006. And, since it doesn't have an installer or the DLLs readily available, you'll waste a lot of time looking for that too.
You might consider trying this tool. Looks pretty good to me so far. And, yes, it supports a few languages. And line numbers. And highlighting.
Public Class ThisRocks
Public Function URL As String
Return "http://www.amergerzic.com/post/WLWSourceCodePlugin.aspx"
End Function
End Class
1: public ThisIsOkToo
2: {
3: public string URL()
4: {
5: return "http://www.amergerzic.com/post/WLWSourceCodePlugin.aspx";
6: }
7: }
A word of caution. There's a little silliness around seeing the formatted code properly in WLW. You'll end up getting used to switching to the HTML source and back... or just learning to trust the tool.