HARDWARE
Mostly, I use trailing edge hardware technology that I can buy second-hand and cheaply on EBay.
When coding, I most frequently sit with my feet up on my desk and my keyboard on my lap; hence, while I own several desktop, laptop, notebook, and netbook boxes, my main dev box is a notebook that I bought for $199. In particular, the notebook's a Dell Latitude E6430 (Intel i5-3210M CPU - 2 cores [4 threads] running at 2.5 [up to 3.1] GHz with 8GB RAM). The highest base screen resolution the notebook natively discloses is 1600x900. I use xrandr (X Rotate AND Resize extension) to drive the resolution to 1920x1080 (1.2 times 1600x900). I don't always attach my second screen, but it's an Acer G206HQL 19.5" that I run at 2400x1350 (1.5 times its highest base 1600x900) for browsing/testing with developer tools active.
When travelling for pleasure (hypothetically NOT on call except for emergencies), I prefer to carry a Dell Latitude 2120 Netbook (Intel Atom N455 - 1 core [2 threads] running at 1.66GHz and 2GB RAM). The screen is 10.1" at a resolution of 1536x900 (1.5 times its highest base 1024x600 - see note at end). Got this netbook for $25 (yes, that's twenty-five dollars US) on EBay. Mine originally had Windows XP installed, which I immediately replaced with Linux (currently Mint 17.3). These boxes are really sweet for travel. I've also got an extra-nice ASUS (eee pc) netbook with the same processor and memory as the Dell. The ASUS weighs only 2lb - the Dell weighs 3lb. The partially ruggedized Dell is a more durable traveller, and I'm not nearly as concerned with losing my investment, if something awful happens, e.g., TSA or ICE seizure or some other form of theft.
For dev (and other, local, home network) purposes, I run a print/file/SVN server on a LENOVO ThinkCentre M58p (Intel Core2 Duo CPU E8400 - 2 cores at 3.00 GHz and 4GB RAM). I bought that box for $65 on EBay (no OS or HD, since I had drives and wanted to install and configure my own OS). The server runs headless, providing access via ssh (I can start an X-server and run ssh with the -X (X forwarding) option, if I need more than a command-line interface.
SOFTWARE
Largely, I use Open Source software. For my own computers, I've been been running at least one box with some flavor or other of Linux since 1994. I had to use Windows professionally when I was a captive corporate employee, but gave it up as my main, personal desktop in 2008 (moved to Fedora Werewolf).
Since making my switch to Linux as my full-time OS, I've still reliably kept at least one up-to-date Windows box around the house. My sweetie simply "don't be lovin' the Linux"; she made a valiant attempt with KDE/Ubuntu for a year, but her workplaces are ALWAYS Windows shops, and Linux is still not-ready-for-prime-time for normal humans. This gives me a contemporary Windows box for testing.
Currently, the OS for most of my boxes is some release of some distro of Linux. My preferred desktop is KDE. My file/print/SVN server runs KDE4 (when X is active) on Mint 18.1. My primary dev notebook runs KDE4 on Mint 17.3.
My main dev environment is Eclipse Classic (Oxygen 3) for JEE with additional plugins for Web Tools, PHP, and SVN. I began using IBM's WSAD (Eclipse-based) for Java development back around 2000 and personally moved to pure Eclipse shortly thereafter. I like Eclipse and know it better than any other dev platform I've ever used.
Locally, I run a LAMP stack, since I've lately used only PHP for the server side of things.
For text editing, I use Kate, Gedit, and nano, depending on the chore and context. For instance, when editing a config file over ssh on a headless server without a running X server, nano to the rescue.
For graphics, I use Kolourpaint - simple, lightweight, and good for small, quick image manipulation - and the GIMP - good for everything else.
I still rely mainly on Firefox, both for my personal browsing and my initial testing and debugging for web projects. I also have Konqueror and Chromium (Open Source Chrome) on my Minty dev notebook. For testing, I have XP and Windows 7 VMs running on VirtualBox with IE, Firefox, Opera, and Safari. I employ my bride's Windows 10 box (another $199 notebook of the same model as my primary dev box but with Windows 10 as the base OS) for native testing of the last IE, Edge, Opera, and Safari browsers. For testing, I support the latest and latest-1 versions of the various browsers.
For small-screen testing, I have an iPad mini and a Moto G5 Plus and can run my netbooks at their native 1024x600 resolutions. I can also set any Linux desktop's resolution to anything via xrandr.
On my dev box, I run the latest LibreOffice suite. On the Win10 box, I have Office 2013 Professional Plus for those times when only the evil empire's Office tools will do.
Both of my netbooks will do everything that my main dev box will but very much more slowly, since the netbooks' CPUs are less than a third of the notebook's, and there's only a fourth as much RAM.
Richard
NOTE: I can't emphasize enough how much more wonderful a netbook can be when xrandr is employed to crank up the resolution (pretty much any height greater than or equal to 768). Do yourself a favor - get a cheap 10.1" netbook with 2 GB RAM (the max.)and install Linux. Add xrandr, if not already installed and add a startup script to reset your screen's resolution (plus, it's fun to watch your screen resize as your user account loads).
Run xrandr without arguments to identify the logical name of the output target (screen) for your netbook (or other box)...my Dell's is LVDS1. The bash script that I run at startup is:
#!/bin/bash
xrandr --output LVDS1 --mode 1024x600 --panning 1536x900 --scale 1.5x1.5
Enjoy!