You will require [msysgit](http://code.google.com/p/msysgit/downloads/list) or git installed with cygwin. Alternatively I would suggest installing [GitHub for Windows](http://windows.github.com/) and checking out the repo with that.
You will also require [Apache Ant](http://ant.apache.org/bindownload.cgi). In Windows you will want to download the ant zip and extract the contents to somewhere like C:\Ant. Then add the bin folder inside ant to your environment path.
In **OSX**, git is typically supplied. Otherwise it can be installed through the apps thingy that OSX has (Obviously I know very little about OSX but you want the dev tools stuff).
The linux git can be fetched from any package manager. If you're on a Debian based machine (This includes **Ubuntu**) you likely have aptitude, so the command would be ```sudo apt-get install git```.
If you're running a RedHat based system, such as **CentOS** or **Fedora** (or some other Distro I've never heard of), then you might have the yum package manager ```sudo yum install git-core```. In the case that this does not work through yum, you might not have the packages added to yum to be able to find git. I wont go in to these here, but if you have issues feel free to contact NeverCast in #OpenMods on irc.esper.net.
Ant can be downloaded from the same place as the Windows Ant, as it's just a Java application. I assume that if you're using Linux, you're proficient enough to work out how to get that working, and if you're developing under OSX I assume you can work it out as you're brave enough to be developing in OSX :P