As of vendors - whose bootloaders are easy to unlock, and whose not? The vendors differ in their attitude to unlocking the bootloader. Some require codes for unlocking, some not. Some require proprietary, Windows-only software, other not. 

Which of the vendors are friendly to people unlocking the devices and which not? Which of them require obtaining tokens, which require proprietary and not-cross-platform software? Which treat unlocking the bootloader as voiding the warranty?

This is only devices which can run Android.

(I started the community wiki)
Please fill in the gaps!

Methods of unlocking

Unlocked by default or not locked at all


Fairphone
ZTE Nubia


Unlock with fastboot, no token needed


Google (excluding Verizon variants)
Huawei (very old models)
OnePlus
Xiaomi Android One devices (Mi A1, A2, A2 Lite)


Unlock with fastboot, token needed


Huawei (some models, mostly "moderately old" ones)
Moto (excluding carrier or Amazon Prime variants)


Unlock without fastboot, no token needed


Samsung (some models, often non-Snapdragon and non-carrier variants; requires enabling OEM Unlocking in Settings, or using official CROM Service app)


Unlock with proprietary software only, token needed


Xiaomi non-Android One devices (Windows-only software, unofficial flashers for other OSes - but still closed-source)


Note:  There are some important changes regarding unlocking bootloader application status and reason on MIUI based devices:


  The process is  now simplified and the permission is granted
  automatically. You don't have to fill in the application form or
  wait for the SMS anymore. Login and head towards
  http://en.miui.com/unlock to download Mi Unlock Tool


Important: Bind your Mi Account in Developer options → Mi Unlock and follow the waiting period shown in Mi Unlock Tool

Not unlock-able (unless by exposing a vulnerability)


BlackBerry
HMD Nokia (Can be unlocked with a token, but only a few XDA members know how to obtain it, and they don't publish the method)
Huawei (application for Honor unlock code still in limited availability to XDA users here)


Effects on warranty

Does not void warranty


OnePlus


Unlocking the bootloader voids the warranty