Just an amalgam of handy sites, tools, online stores I use frequently for projects in no particular order. Will probably add to this as I remember handy sites or come across new ones!
KiCad - FOSS PCB ecad Software
QMK Main Site - Open Source Keyboard Firmware
QMK Configurator - Online GUI to customize and build QMK firmware files
Keyboard Layout Editor - Online tool to visually build keyboard layouts
SwillKB - Handy site for developing switch plates and sandwich cases for keyboard layouts imported from the above keyboard layout editor
Keyboard Online Stores
NovelKeys - Great store for switches, and home of some of my favorite contact based key switches. Also runs some group buys
1upKeyboards - New York based mechanical keyboard store, can find most of what you'd need here for basic builds
Clueboard - Custom 60/66% keyboard PCBS and parts, key contributor to QMK
LFKeyboards - Maker of some solid Keyboard PCBs
KBDFans - Keyboard online store, Has lots of nice aluminum cases and all the keyboard parts you need if you can't find em elsewhere, also runs some group buys.
KPrepublic - Another Chinese based Keyboard Company store, Lots of options and keyboard types
sentraq - Offers basic keyboard parts but handy if you can't find them elsewhere
OLKB- Creator of the Planck 40% and Author of QMK, A key contributor to the Mech Keyboard community
MechanicalKeyboards.com - Online seller of many commercial mechanic keyboards as well as parts for boards like the Phantom TKL pcb
PMK - Direct to Consumer facing store of Signature Plastics, A keycap manufacturer based in Washington state, well known for their SA profile keycaps
Misc online Stores
Tindie - Storefront to facilitate makers and hackers selling there own products, very cool to browse and see what's there
Mouser - One of the big 3 electronic component online sellers. Semiconductors, Passives, SBCs and more can be found here. Ships from Texas.
Digi-Key - Another of the big 3 online component sellers. Ships from Minnesota.
Embedded News Blogs
CNX Software - Blogs on embedded products and news related. Lots of SBC and Micro-controller news here
Linux Gizmos - Posts of linux tangetial computing news/products
Liliputing - Mobile and small computer blog posting mostly about products in that space