http://www.adafruit.com/ This site is fantastic for tutorials, resources, all help imaginable in fact. They have videos, live shows and amazing products. They create their own components as well as sell others, and all products are tried and tested (and broken apart) before they sell them. Complete idols here for their hard work, passion and dedication.Recently teamed up with Arduino.cc to manufacture Arduino.cc boards so watch this space too.
https://www.sparkfun.com/ This is pretty much The One of all the component sites. It is usually where I go to look up components to see their descriptions or for more information and links. They often have tutorials or datasheets and it is a great place to see what components are even available because chances are Sparkfun has it first and then the other distributors get it from them. USA based site so if you’re in the UK like me be aware of hefty duty fees! Ouch.
This site has very cheap components for sale if you search hard enough.They will take up to a month to be delivered though so you have to plan ahead. Also – if you buy “Arduino” boards from them,it’s really cool if you also donate $1 to Arduino to support their making / open source files (as the other Arduino official sites include this in their pricing).
http://www.bitsbox.co.uk/ Loving this site for all those little odds and ends. They have a great selection for the smaller stuff – screws, wires, nuts, electronics, resistors, solder etc, I save up the things I need and then order a bunch of stuff on here. Really great prices and not all the typical items.
http://www.skpang.co.uk/catalog/ I really like this site for their large amount of wearables components. They have a super collection of items that are harder to find on other sites. I typically go to them for wearable / sewable stuff. Also – if you are buying more than one of a component, it can sometimes be worth emailing them first as they do offer discounts which is great. Quick delivery too.
http://www.coolcomponents.co.uk/ Great place to get components. Often they have stuff that I can’t find elsewhere and I’m finding it difficult to get a hold of batteries and they seem to be able to get a hold of them. Fast delivery.
http://proto-pic.co.uk/ a great retailler for all these types of components and wires. There is always fast delivery and the packaging is fantastic – you get these wonderful plastic boxes for packaging which I keep and use for projects all the time. It’s one of the main reasons I often shop here over other sites. Also, don’t forget your student discount if registered with your student account.