Since upgrading to Snow Leopard, I noticed the problem that the Open Recent menu item in my applications is always empty. I had to search quite hard on the internet to find the solution so, if you’re having the same problem, here it is. Unfortunately, you have to apply this to each application you want to fix.

1. 1)Browse to /Users/<yourname>/Library/Preferences/ for a list of .plist files. In this directory, you need to find the name of the application that you want to fix. In my example, this is com.apple.iWork.Numbers.
   

2. 2)Open a console window and type:
   defaults write <application> NSRecentDocumentsLimit X
   This makes your application remember X number of recent items. For example:
   defaults write com.apple.iWork.Numbers NSRecentDocumentsLimit 5
   This makes Numbers remember 5 recent items.
   

3. 3)Restart your application and it will start remembering recent items.
   

Update:

I discovered that the option in System Preferences -> Appearance -> Number of recent items -> Applications had been reset to zero. Setting this to a value seems to fix every application - as expected really.