<<< I want to run some software on it which needs OSX, is a G3 likely to have this on? The person who offered it me said they could put tiger on it, is this better? Is it an OS at all? >>>
Mac OS X version 10.4 is "Tiger" and this is the very latest operating system.
<<< It will not come wit a monitor, is there any kind of "splitter" that will allow me to use this and my PC on the same monitor? >>>
In this respect it shouldn't be any different from any other "PC", a KVM switch should let you share a monitor. I actually have a VNC application installed in the Mac so that I can "remote control" a Windows PC.
<<< I understand it's UNIX based, does this mean command line stuff or does it have a full GUI? >>>
Yes, it has a full GUI but it is probably more like Linux than Windows.
<<< Can it be added to my existing (two windows, one linux) network? >>>
Absolutely, I have Mac, Windows and (SuSE 10) Linux networked via a router, all online and file sharing.
<<< I have been offered an apple macintosh free of charge. >>>
Go for it and have fun!