Unwrap Tools (Chuggnut) (Amit Singh)
This handy free script adds extra tools to the Unwrap UVW modifier such as: Rotate 90' or 180', Align verts vertically or horizontally, Flip & mirror islands, Space verts evenly, Taper, Skew, etc... When working in the unwrap window, this bitch is a must have!

UV Aligner (Ammar Almakzumi)
This is the best script I've found in a long time! It's sole purpose is to align verts in the Unwrap UVW window, similar to what 'Unwrap Tools' can do. The huge benefit being that UVAligner can straighten an edge selection without having to convert it a vertex selection first! No shit you say? That's right my friend! This baby can even do a loop and align in one click! A major fucking time saver and definite must have!

Advanced Painter (Herman Saksono)
This awesome script by Herman Saksono has been around for years, and gives you the ability to paint objects on other object's in Max. It comes with presets like grass & hair, and even stones & toothpaste! but where it truly shines is it's ability to paint with any custom object you want. Simply pick an object and start painting! Loads of options give you the ability to randomize size, orientation and distance easily.

Wireframe Renderer (John Martini)
Found this script the other day! It does exactly what it says, lets you render out a model's mesh overtop of it's surface. Pretty much the same thing as Vray's edgetex material, but this uses Mentalray instead. Simply pick the surface, wire and background colors and hit render. MR settings are already applied, so there's no need to spend time tweaking shit out. Also has the ability to set wire thickness...pretty handy!

FumeFX
This voxel based, fluid dynamic plugin for Max allows you to create ultra-realistic fire and smoke FX's. Everything from simple candle flames to complex particle driven systems like explosions, flame throwers, smoke pillars and nukes. Also works in conjunction with PFlow, Thinking Particles, Krakatoa, etc.. If you're interested in getting in to FumeFx or just VFX in general. I'd highly recommend you check out Allan Mckay's website http://www.allanmckay.com. He's one of the premier VFX guys out there and offers many free video tutorials. As well as multiple training DVD's for production visual FX..

Afterburn
Another fire & smoke plugin for Max. Afterburn's been around for a long time and was the go to choice before FumeFX came along. That's not to say it's obsolete, it still has it's many uses. Things that FumeFx can't do, or at least doesn't do well. Mostly used for smoke pillars, missile trails, burning debris and the like. Once again, check out Allan Mckay's tutorials and training DVD's for more info.

Rayfire
This is an awesome script based plugin that lets you completely break, shatter and generally fuckup an object in Max! Before this came along, if you wanted to destroy a model. You'd usually have to model it from a bunch of separate pieces. (with no penetrations!) and hope you could somehow constrain them together so you could break it later with reactor! I think it's safe to say this didn't usually work well. With Rayfire these problems have been eliminated. It doesn't matter how you model something, or how many pieces it's made from!. I'm not going to try and list all of it's options and features here, but If you have a few minutes. I say head over to their website to see just how awesome this shit really is. http://rayfirestudios.com

Krakatoa
Originally developed at Frantic Films. Krakatoa is a Volumetric Particle Rendering system used to render enormous amounts of particles. Krakatoa can cache out millions of particles to disk, saving precious memory and resources in 3ds Max! Multiple cache's (partitions) can be saved out and then re-loaded and combined as one. Letting you render a truly insane number of particles! (Even on a single machine) Epic!