How to Get What You Want is a wonderful resource full of creative fabrication ideas for sensors and actuators, curated by Mika Satomi and Hannah Perner-Wilson.
Five Hundred and Seven Mechanical Movements was an 1868 book by Henry T. Brown featuring 507 drawings of mechanical linkages. The website 507movements.com reproduces these drawings and adds animations to some of them which make their movements easier to understand. The original text (which includes narrative explanation of all of the drawings) is in the public domain and can be browsed or downloaded here on the Internet Archive.
Fritzing is an open-source electronics documentation program. You can draw schematics, breadboard views of electronics, and even design PCBs to be sent out for professional fabrication.
Adafruit is a great educational electronics company out of New York City. They manufacture and sell their own products (we carry their NeoPixel Ring, for instance) and are a reseller as well. They generally have excellent documentation and tutorials!
SparkFun is an education-oriented electronics company out of Boulder, Colorado. They have a broad variety of components and often have good documentation.
Pololu is a hobby/educational electronics company out of Las Vegas. They sell lots of parts including a broad selection of motors and other actuators.
Mouser and Digi-Key are both enormous electronics supplies houses. They have catalogs of millions of items, and will likelier than not carry any fundamental component you need (such as a specific resistor, transistor, LED, integrated circuit, etc.). A note about availability: we use many pre-assembled PCB boards in our lab, such as the ADXL335 accelerometer—Mouser and Digi-Key won’t likely sell an assembled board like that, but rather just the IC that is the brains of the part.
Arrow is another big electronics supply house which (currently) offers free overnight shipping on any order. Daily shipping cutoff is 9 p.m. Eastern.
McMaster-Carr sells an enormous range of hardware (such as nuts and bolts) and raw materials (sheets of aluminum). They have an excellent search and overnight shipping at low cost. Daily shipping cutoff is 6 p.m. Eastern.
Grainger is another hardware giant with a huge range of parts.