Farnell: Good selection of individual electronic components in the UK, but smaller suppliers may be cheaper for some popular items.
CPC: Sister company to Farnell. Good for tools, general electrical and hobby electronics. Again, some items might be cheaper elsewhere.
Bitsbox: Basic (traditional) components for hobbyists. Low delivery charges and good service.
Phenoptix: UK-based hobby electronics. Quick delivery with a low-minimum order value. No longer selling parts! Shame
Sites with free samples. Some require a student email address.
Microchip: PIC processors, EEPROMs, and a wide variety of other digital electronics
Texas Instruments: Analogue and digital semis, sensors, etc
Hobarts: Laser-cutting materials.