The Raspberry Pi  is a "a tiny and cheap computer for kids". Cost: 35 dollars to start. Make Magazine  is a great hobbyists sharing site. A great first place to look to see what people are doing.
- Product Review: The Raspberry Pi has been reviewed by Pc Magazine 
- Temperature sensor: Example of a Raspberry-Pi-interfaced temperature sensor  at the Computer Laboratory at the University of Cambridge .
- Flow sensor: See the discussion at Raspberry Pi . Lots of interest; probably not implemented yet.
- Electrochemistry: See the "Low-Cost Portable Potentiostat for Biosensing Applications" by Elliot Friedman (esf59) and Alexander Hartoto (akh75) at Cornell University , the open-source potentiostat at Public Lab , and the "WheeStat5.1" at OSH Park .
- Motion control: The audio latency has been estimated to be approximately 10 ms for the Raspberry Pi . The latency is the time to get an incoming signal digitized, manipulated, converted back to analog, and sent back out. If you want to do really fast feedback, then the latency determines how fast of a stable feedback loop you can make.
The Arduino  is an "Open-source electronic prototyping platform allowing to create interactive electronic objects." Cost: 50 dollars to start.
- Explained: "What Is Arduino & What Can You Do With It? posted on 2011/09/25 by James Bruce . "To you or me, it’s like a little computer you can program to do things, and it interacts with the world through electronic sensors, lights, and motors." See his links to YouTube videos sureveying what has been done with the Arduino.
Raspberry Pi, Arduino, and accessories for "Making" are available at the "pimoroni" shop 
I would use an analog multiplier chip to make a (cheap) lock-in amplifier and/or an AM radio demodulator.
Alan Carnagey has posted to YouTube an hour-long video comprised of short segments showing real and bunk science . Cany you tell which segments are real science and which are bunk/junk?