Here are some of my favourite links:
Automation Direct – They have their own line of affordable programmable logic controllers. I bought everything from them to build my Programmable Logic Controller Trainer Panel. They’re based in the USA and have a great variety of automation parts.
Dipmicro – They’re based in the Niagara Falls area on both sides of the border. They’ve got a good variety of components, as well as prototyping stuff (breadboards, perfboards, wire) and some Arduino compatibles and peripherals. I like them a lot because their prices are very reasonable. Standard shipping is under $4. Everything comes in a padded envelope and usually takes 3-4 days to get to me through my Canada Post mailbox. Priority shipping may be required for heavy orders or items that are too big for the padded envelope. Those orders come in a box and pretty much arrive the next day for me.
Dirt Cheap Dirty Boards – An inexpensive PCB manufacturer.
Robot Shop – This site caters mostly to the mobile robotics crowd so I don’t buy from them very often. What I have purchased from them so far has been of good quality, which is something that can vary at times from cheap sources. I wish they would offer cheaper standard shipping for Canadians like Dipmicro as their only method of shipping is expedited, costing around $6.
Sayal Electronics – This is the only physical electronics store I regularly go to. Their prices aren’t really the cheapest (understandably, gotta keep the lights on somehow), but they’ve got a great inventory of electronic components and other items I’d rather buy in person, like solder and flux. It’s great to go there if I feel the need to get a part quickly or even just walk around to get ideas.
Tutorials and Reference
Adafruit – Look for help with any Arduino part and you’ll probably end up on the Adafruit Learning System.
Instructables – I have used articles on Instructables to get inspired and started on some projects.
Makers, hackers, artists & engineers community on Google+ – This is a great place to share projects and see what other people are making.