Black Country Atelier in the Jewellery Quarter of Birmingham offer workshops on Arduino, 3D printing, laser cutting etc. They also offer services such as laser cutting and printing.

Education charity promoting STEM education to school pupils. A supporter of the Rodni project.

The home of Arduino. Here you can download the Arduino software and find lots of information.
There is also an excellent reference section.

Oomlout is an excellent source for Arduino hardware and a good selection of sensors and other electronic parts. The Oomlout Starter Kit for Arduino is a brilliant starting point.
You can also download the Oomlout experimenters guide free of charge. This includes many useful example circuits and programmes.

The instructables web site has many interesting step by step projects based on Arduino

The Make magazine blog is a good source for information on Arduino projects.

ebay is a good source of Arduino sensors and electronic parts. Many are supplied from China and Hongkong so delivery times are up to a month. My experience is that the suppliers are reliable. Servos can be sourced at an exceptionally good price.

The Arduino Cookbook by Michael Margolis is a comprehensive guide to programming Arduino.