LINQ To SQL Designer issues
Today I was trying to load a LINQ to SQL data context in my project and every time I double-clicked to open it I received this message:

After trying to sort it out for a while I finally deleted the values under this registry subkey:
HKEY_CURRENT_USER\Software\Microsoft\VisualStudio\9.0\Packages
Seems that Visual Studio had decided not to load my designer tool on startup and marked it to be skipped next startup.
After deleting these values, everything now works again!
Hope this helps,
Jason