I have a Windows 8 tablet running the full W8 (read: NOT RT). I know that there are apps for iPad (not sure about Android) that will let your tablet function as a secondary monitor. Is there any way I can use my W8 tablet as a secondary monitor to my W8 desktop?
Since your tablet is running Windows 8 and not Windows 8 RT the fact that it's a tablet is not really relevant here. Same goes for Windows 8, likely anything that works for Vista and 7 will work for 8.
With that understanding, this answer that suggests tricking Windows into creating a virtual monitor, and then using a VNC program to view it should do fine. The VNC program would run on your tablet.
ZoneScreen appears to implement this as a turnkey solution, and might do what you want:
But caveat user:
The functionalility is not going to be exactly like dual monitors, but there are programs like:
What both of these do is let you share your keyboard, and mouse between two systems using their own displays. It is a great facsimile to extending your your monitors through a seconds system.
For me having tried Synergy and Input Director, Input Director is far superior for Windows only machines. Both are functionally equivalent and Synergy is the best option for any workspace where you have multiple operating systems that you want to synergize.
ID is dead simple to set up and I haven't had any issues with it while Synergy has yielded perpetual connectivity issues.
Latest version: http://www.inputdirector.com/downloads/InputDirector.v1.2.2.zip
Install guide with lots of pictures: http://www.inputdirector.com/quickstart-install.html
Synergy if interested: http://synergy-project.org/