IIS, ASP.NET: Could not load file or assembly 'System.Core, Version=3.5.0.0, Culture=neutral' or one of its dependencies'

If you receive this error when trying to access a newly deployed application on IIS:

"Could not load file or assembly 'System.Core, Version=3.5.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089' or one of its dependencies. The system cannot find the file specified"

Then check that you have the .NET framework version 3.5 installed. This was the LAST thing I checked after about a million other things - I wish I had checked that first!

I'll know for the next time though!