Apart from having a decent pc system (there's always room for improvement) and a dual monitor set-up I use some extra input devices next to my keyboard/mouse set-up:
3D mouseAwesome, quick and easy to use, universal 3d navigation in every program. I use a 3d mouse from 3D Connexxion everyday. Got this one 3 years ago as a present and I'm still very very happy with it. Fast and smooth navigating in your viewport without the strain on your wrist holding down Alt, middle mouse for jerky inaccurate rotations around your model. Also helps again repetitive strain injury.
Wacom tabletMust have, best piece of equipment for sculpting and texturing. I currently use an Wacom Intuos 3 A5 Wide at this moment. There are other brands that supply tablets but nothing can really compete with the robust quality of a Wacom. As with the 3d mouse it will save you some RSI.
Great, cheap addition for tablet users, removes friction and keeps your tablet clean. I know it looks like something Micheal Jackson would wear, but really, you should try it.
Realtime viewport shaderAfter baking out my base textures I like to work on the texturing whilst checking my progress in the max viewport. For this I use a realtime viewport shader that shows me how the model will basically look inside a game engine. Rendering out your models and texture updates isn't really representative of how it will look in a game engine and takes way too much time. I can really recommend Xoliul's Viewport shader made by Laurens Corijn which comes with a ton of options and is also great for setting up final presentation shots.
Xoliul's Viewport Shader can be found here
And a quick and simple video on how to set it up can be found here
Meshlab - FREEGreat tool for optimizing meshes, reducing polycount and a bunch of other functions. Stand-alone application. Although if you have acces to 3ds max 2010 and above I recommend using the Pro Optimizer modifier found under your modifiers tab. More info on how to use it can be found at the Eat3D site in a quick free tutorial video
Software can be found here
nDo - FREEA set of Photoshop functions great for generating normalmaps, it has a great bunch of options to help with your normalmap/texturing workflow.
nDo can be found here
With some great getting started info here
Nvidia Normalmap Filter - FREEIs a bit older and can have some compatibility issues with more recent versions of Photoshop but nonetheless is a great helper in your normalmapping/texturing workflow.
The Nvidia tools can be found here
Xnormal - FREEA stand-alone application that can be used to bake out your maps from your highpoly models. Also comes with a bunch of extra features that help in your texturing workflow, these are available in the application as well as a bunch of options under the Photoshop filters. Xnormal works very fast compared to native baking options in 3ds Max and Maya. Andy Davies over at Eat3D did a great video about using Xnormal found here
Xnormal can be downloaded here
Crazybump - PAID - trail versionStand-alone application that can generate a variety of maps with realtime preview options and precise control through sliders. Also the option to copy/paste from and to the clipboard make it very easy to use in combination with Photoshop. Crazybump can be found here
Marmoset Toolbag - PAID - trail versionThe Marmoset Toolbag is a great application for getting those great presentation shots of your models and small scenes. It has a lot of options for setting up different materials, lighting conditions and post-proces settings. You can quickly output high resolution screenshots of your model and even render out a turntable animation of your model. The Marmoset Toolbag can be found here
Max scriptsFunctionality for 3ds Max and Maya can be further expanded by using custom scripts made by yourself or others. Polycount recently posted a great overview of some of the most useful scripts for 3ds Max available found here
So, this is my short list of handy tools I use on a daily basis. A lot of people will already be familiar with most of them but if you're not, I'd recommend checking these out. Also software will continue to evolve so always be on the lookout for new things that might make your everyday easier.
I'm sure in a while this list will be updated, if there's anything I should try, please let me know.