The only free software that i use for theatre is LD Calculator lite. It is very useful to figure out different things.
The things that it calculates are: Photometrics
Electrical Base Laws
Electrical Phase Loads
Electrical Load List
Electrical Voltage Drop
Resistance Color Code
Weight Load List
Truck Laod and Volume
General Converter DMXAddress
It is very useful especially if your show is going on the road, but in most cases you will only need it to calculate half of the things it is capable of doing.
Other types of software that I use cost money. They are also really good, if you can afford it, or if your school has an extra lisence of the software.
For lights I use VectorWorks
For construction and all the different types of plots and plans I use AutoCAD
For sound I use Cool Edit Pro.
I did not have to pay for any of these because I the theatre had liscenes to all these programs. I would sudgest that if you want these things free, see what the theatre or school has that you can use.
There are several free cad programs out there. They're better than nothing. I'm also a huge fan of Sketch-up even the free version is extremely powerful and extremely easy to use. Google it learn it use it.
Just about all the theater consoles now have a free off line version of their software that you can download and use on your home computer. Just check out the various manufacturer download pages. Most of these require you to either own the actual console to upload it or to buy an expensive attachment interface from them (commonly called a dongle) to make it work.
Virtual Smartfade doesn't build shows like the other offline editors, but it really gives you a feel for the operation of the console. I was very skeptical about the idea of a low-budget, two-scene based ETCconsole until I sat down with VSmartfade...it's actually pretty nice.