Integrated Circuits
The digital Potentiometers made by analog devices (AD5204) has 255 positions can be adjusted by the microcomputer that can receive commands from the computer. This is a great way for analog circuitry to have digital control. This is a chip with 4 digitally controlled POTs and the pots can be daisy chained to have multiple chips controled by a single SPI interface.
Digikey Part Number: AD5204BN10-ND
AD5204 - 4-Channel, 256-Position Digital Potentiometer
http://www.analog.com/images/Product_Descriptions/40826AD5204_S.jpg
Datasheet: http://www.analog.com/UploadedFiles/Data_Sheets/140808745AD5204_6_0.pdf
Availability:
- Free samples: http://www.analog.com/en/prod/0,,761_797_AD5204,00.html#price
- Digikey: http://www.digikey.com/scripts/DkSearch/dksus.dll?Detail?Ref=1480&Row=284443&Site=US (10k version - $5.02)
Eagle footprint data:
Eagle library (footprint, schematic symbol)
voltage regulator
LT1528 High current, Variable Voltage Regulator
TPS V-Regs These are 3.3V and 5V LDO, Low-Noise Voltage Regulators. Very small SOT-23 SMD package. 150mA max current. Best used in battery applications.
Basic Voltage Regulators Variable voltage regulators, set output regulators, we give you the whole breakdown. Perfect for use with an external wall-wart power supply.
microcontroller
LPC2103 Low cost 70MHz ARM7TDMI-S FLASH microcontroller from Philips.
Op Amps
op amps: great for amplifying weak signals from sensors to a more useful level. Also used in filters, integrators, etc.
LT1114 - Dual/Quad Low Power Precision, Picoamp Input Op Amps
http://www.linear.com/pc/images/products/thumbnails/1365.jpg
Datasheet: http://www.linear.com/pc/downloadDocument.do?navId=H0,C1,C1154,C1009,C1100,P1365,D1714
Availability:
- Free samples: http://www.linear.com/samples/orderSamples.do?navId=H0,C1,C1154,C1009,C1100,P1365
- Digikey: http://www.digikey.com/scripts/DkSearch/dksus.dll?Detail?Ref=9250&Row=451270&Site=US ($8.80)
Schematic symbol:
Available packages: 
Eagle library (footprint, schematic symbol)
