UltraMon by Realtime Soft
In my experience, once you go "dual monitors" you never go back. I first started using a multi-monitor setup back in 2003, and it has been a requirement ever since. Of course, these days it is much easier since laptops are more expensive than they used to be, so you one need one unattached screen. In the past it was quite a task to convince your company / client you needed not one, but two screens. However, using multiple monitors and moving from machine to machine can be difficult and annoying because of the variation between graphics cards and software to support the setup. Windows can control desktop extension consistently, but it is very limited feature-wise. And that is why I love UltraMon. It lets me use all my favorite features to span two monitors across any graphics card. My favorite feature is that you can extend the start menu across the screens, and it can function independently for each monitor. It also lets me keep my hotkeys to swap screens consistent. Fair warning again: If you go two monitors you will never be able to go back to one without feeling crippled!
Notepad ++ by Don HO
I am one of those coders who prefers, for many things, to write code in a plaintext editor. I type 100 WPM people, I'm faster than autocomplete. For years I put up with Windows's terrible notepad.exe which has seen virtually no changes since 1990 (seriously). I mean, the so-called features are "Word Wrap" and a status bar that I can never get to appear. Notepad ++ is basically a drop-in replacement that has tabbed windows, text manipulation, programming language color coding, and so many features that there seems to always be an updated version to download. While I do so much more of my programming in IDE these days I still rely on Notepad ++ for so many things like batch programming, deep searching, HTML editing, and the list goes on …
Paint.NET by dotPDN
Adobe Photoshop is the best software out there for image editing … But man is it expensive! Sometimes it is simply not available for use when and where I need it. And how many times do you get stuck having to create your own images because there is not a Web or graphics developer on staff? When it has not been an option for me, for years I turned to GIMP but it has never offered an experience similar to Photoshop. For those of you who have used it, you know what I'm talking about! Paint.NET, however, is about the closest thing you can get to a Photoshop experience in a free package. It is not perfect – quality in gradients and filters are not as good as Photoshop and there are some missing tools (where the hell is the "feather" option ?!) but overall it offers a great little package for image editing.
What software can not you live without?