All sound designers have their toolset. For some it's a Pro Tools HD rig, tricked out as far as possible. Others are comfy with Sound Forge and a few plugins. Below is a list of some of the tools I find myself using on a regular basis, as well as some discussion about them. In time I plan to share some of my tricks and shortcuts.
I've been a Steinberg guy for quite a while. I like the interface and the ability to work in surround without breaking the bank. Using a PS2 controller as a surround panner is also a pretty rad feature. This is where I spend about 90% of my working time. Oh, and I'm still on 2.0. It's comfy and stable, no need to upgrade :)
Sony Sound Forge
The de facto sound editing tool for most PC users. It's great for final tweaks to sounds before shipping them off. It's also good for editing and organizing field record sessions.
I'm not really what to say about this one. It's a great tool, but quite odd. Don't let the "music studio" description fool you, it's very useful for sound design. Some of the granular processes it has are unbeatable for sci-fi sound design. I used it for lava bubbles on the DS project.
Software : Plugins
GRM Tools Classic and ST Bundles
The GRM Tools are amazing. They're great for anything weird you want to create: UI sounds, futuristic creatures or weapons, etc. They can also be used for creating nice sweeteners for more realistic sounds.
DelayDots Sound Designers Spectral Suite
This is a fairly recent purchase, and I'm still getting the hang of it. I used the Spectral Morpher for some odd item pickup noises and it performed pretty well. It can quickly get out of hand though, so watch your monitors.
I wouldn't go so far as to call these a requirement, but most people I know have them, and I can't really get through the day without them. The Waves plugs are my workhouse plugins. They're clean and very functional, and I know I can rely on them.
I've owned this unit for about five years and it's never given me even a hint of trouble. It's only 24/48 (and should probably be swapped for the mkII model), but it's a fantastic interface with decent conversion and stable drivers. I use this every single day and love it.
This mic is my baby. It goes almost everywhere with me. I can point it at just about anything and get a decent recording. I used one when I worked for GTN in Detroit and fell in love. My particular 418 has a bit of history: it was used quite a bit on the ill-fated Starcraft: Ghost project. I have a full Rycote kit for it, and am rarely more than five feet away from it.