Chrome. I mentioned this in the install post. I prefer Chrome as my primary web browser, in large part because I've been using it for years and thus have it set up the way I like with add-ons/bookmarks/preferences/etc. I could do the same with Firefox or Microsoft Edge, but why go through all that again?
Notepad++. My favorite Windows text editor. So much nicer than Notepad (which is true of almost any text editor), with plenty of plug-ins available to add most any text-editing feature you might want.
PuTTY and XMing. If you access Unix-flavor machines on a regular basis, these are invaluable utilities. PuTTY provides terminal access, which I use for most things, and XMing allows remote execution of most X-Windows applications on your desktop.
mp3tag. I have a good-sized library of mp3 music files, mostly ripped from CDs many years ago. In many cases, they aren't properly tagged for Kodi or other music players to categorize them. Eventually I'll have them all fixed, but until then, it's useful to have a simple, easy to use utility like mp3tag.
Turn off Cortana and web search. I like having a search bar for stuff on my computer right by the start button, but I don't want to see Bing results, and I certainly don't need Cortana trying to guess what I want. Getting rid of both speeds up the searching.
Remove recently accessed files/folders from Quick Access. If I want something showing up every time in my Explorer windows, I'll pin it there. No need for Windows to try to be helpful.
Remove the old Windows 7 boot entry from the startup menu. Since I had a Windows 7 install at the time that I installed Windows 10, it helpfully gave me a startup menu whenever I reboot, allowing me to choose my OS. Useful at first, but once I decided Windows 10 was working, I don't want the Windows 7 option any more. The top answer here describes the process to remove it (via an elevated command prompt).
Turn off hibernation. My machine is a desktop, no need to take up hard drive space or processing time maintaining a hibernation file. Open an elevated command prompt, and enter "powercfg -hibernate off".
Update the privacy settings. There's a lot of stuff that Windows 10 will report to Microsoft, or app owners, or websites, etc unless you tell it not to. This article has a pretty good summary of things you can easily turn on or off. Everyone has a different level of comfort with what information they're willing to share. There are also things you can't turn off, which I find to be poor customer service by Microsoft, but doesn't concern me unduly.
Make a shortcut for sleep mode. I like to put my computer to sleep when I'm not using it, rather than shut it down. It's mildly annoying to have to go into the power menu and choose sleep, though. I could set the power button to trigger sleep, but I can't easily reach it with the way I have my desk set up. So I created a shortcut on the desktop that executes "C:\Windows\System32\rundll32.exe powrprof.dll,SetSuspendState", so all I have to do is double-click that to put the machine to sleep.
Make a shortcut for sleep mode. I like to put my computer to sleep when I'm not using it, rather than shut it down. It's mildly annoying to have to go into the power menu and choose sleep, though. I could set the power button to trigger sleep, but I can't easily reach it with the way I have my desk set up. So I created a shortcut on the desktop that executes "C:\Windows\System32\rundll32.exe powrprof.dll,SetSuspendState", so all I have to do is double-click that to put the machine to sleep.