I’ve been an electronics hobbyist for some time now and over time I’ve been collecting components. Below an inventory of parts I own:

Boards

1. Arduino UNO 5V [~£13 – overpaid?]. (Arduino Uno R3 Schematic)
2. Arduino UNO 3.3V/5V via selector [~£11]. My workhorse Uno for rapid prototyping and also a programmer for my AVR chips.
3. Arduino Mega 2560 [~£10]. Mega2560 Schematic.
4. Arduino Nano: fits breadboard, integrated USB.  [~£6] Arduino Nano Schematic.
5. Arduino Pro Mini 3.3V (2 of them) [£2-£4]. [No USB chip to draw unnecessary power]. Pro Mini Schematic.
6. Arduino Pro Micro (Leonardo) [£4-£5]. Pro Micro Schematic. Uses ATMega32U4. Advanced USB functionality.

Chips

1. ATMEGA 328P microcontroller [~£2 each – 12 pcs]. (Datasheet)

Displays

1. Nokia 5110 84*48 pixels
2. HD44780 (Hitach) 16×2 character display

Power Supply

1. Step up DC-DC Boost. 1-5 V to 5V (up to 500 mA). [~£1 – slightly cheaper in bulk]. Around 8pcs.

Reference Documents

1. ATmega328 pin mapping
2. ATmega2560 pin mapping