Every developer has their own environment. Some can't simply move from one environment to another without having to set preferences or download specific applications to help them get their work done. Other developers simply can't live without a few applications or tools to help make their life easier. So here's a list of mine and why I can't live without them.
Dropbox. First off, if you don't have Dropbox download it now. Dropbox offers a way to keep a folder of all your goodies synced across all your devices as well as synced in Dropbox's servers so you can access them from anywhere. It's free, cross-platform, and very easy to use. During development, I keep all my work files in my Dropbox. If my desktop's HDD goes, or if I just want to work on my laptop, I don't have to email or transfer the files myself; Dropbox will sync the files without you telling it to do so effectively mirroring your files across all your computers. On top of that, there is version control. This means I can jump back to an older version of a file if I messed something up. Oh- that's right, did I mention you can share files and folders with other Dropbox users? They'll automatically sync to their computers too.
A Notepad. Wait, how are notepads dev tools? Okay- I'll be level with you, I *hardly ever *use paper anymore. The keyboard offers greater accuracy, speed, and value, but I can't just whip out my computer and write an idea down in 5 seconds- it's just impossible. Pen and paper are thought to be a thing of the past, but there is no better way to rapidly prototype several designs. They're portable too! I was on a bus in China and I was able to take out my notepad and write an idea for a project I'm currently working on now; which reminds me, old notepads are awesome to flip through as well!
Google. No matter how long you've been doing the same thing, there are always going to be things to look up. Using Google to search for online reference material is something every developer does one time or another. I feel people don't consider Google a tool anymore; it's just another part of everyday life to most. Apart from searching documentation, Google has a very powerful calculator that can help with things your OS's calculator can't do. To be honest, I actually had a hard time coming up with this one. I fall into the category of people who just see Google as another part of life, not a tool I can aim at specific tasks!
Well, there you have it! Those were the things I can't live without when I'm developing things. The order doesn't matter, they're all equal when it comes to importance. Something else that didn't make it to the list are friends that help me our with testing as well as giving opinions on designs. The numbers would be too large to include them all, and my friends aren't tools.