So you want to break into the video game industry (you want a game biz job). First, you have to know which type of job you want -- if you don't know which you want, you need to read about the game industry and the types of jobs in it. Then you might need to make a decision. Third, you need to be qualified for the job. Fourth, you need to know how to find information and how to ask good questions (you need to not ask bad questions). Finally, we have tips for getting the job.
I've a .NET business progammer background with a hobbyist interest in making my own games - mostly mobile games - but I'm curious to know what a Tools Programmer does and what sort of things I should be working on to aim for a job like that? A lot of the job specs have "plugin development for Maya or 3DS", is there any more to it than that?
I dabbled in tools programming while at Eidos a few years back.
I'd recommend reading up on Managed C++ because you may need to interface directly with game objects which are written in C++, and you can write Managed C++ wrappers around these 'native' classes so that C# code can interact with them.
Also, for the UI, Windows Forms and the Model-View-Presenter pattern is a common choice, but gaining pace is WPF and the Model-View-ViewModel pattern. Could be useful to have knowledge of those two.
I never had to learn third-party scripting, but Unreal Script and Maya plugin development are often listed for tools programmer roles.
"Tools programmer" is a very broad term. It ranges from library over systems to utility programmer. Writing plug-ins, im- and exporters for various other software is also a possibility. It might even refer to build engineer.
Broad programming knowledge is good for any programmer position, as is experience.
If you want to know more details, you could check out job offerings. They usually contain a wish list of skills.