- "Intro/History: Introducing a 100-year-old mechanical computer"  by William S. Hammack (a.k.a. engineerguy . In this video Hammack explains the design and operation of a mechanical computer that performs both Fourier synthesis and, amazingly, Fourier analysis. Thanks to Douglas Natelson at Nanoscale Views  for bringing this beautiful series to my attention.
- 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?
- 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.
There are an increasing number of Raspberry Pi alternatives  . One example is BeagleBone Black , starting at 55 USD. The board offers many more inputs and outputs  and enough on-board storage that you don't need to boot from an SD card.
|||"Raspberry Pi alternatives emerge to fill need for speed" by Agam Shah at PC World; posted Apr 23, 2014. http://www.pcworld.com/article/2147420/raspberry-pi-alternatives-emerge-to-fill-need-for-speed.html|
|||"8 alternatives to the Raspberry Pi" by Megan Treacy at Trehugger; posted June 17, 2014. http://www.treehugger.com/slideshows/gadgets/8-alternatives-raspberry-pi/|
|||BeagleBone Black Rev C - 4GB Flash - Pre-installed Debian. https://www.adafruit.com/products/1876|
|||"How to Choose the Right Platform: Raspberry Pi or BeagleBone Black?" by MMichael Leonard at Make magazine; posted February 25, 2014. http://makezine.com/magazine/how-to-choose-the-right-platform-raspberry-pi-or-beaglebone-black/|
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 
Little Bits are small electronic modules that you string together to do things . They are held together by magnets. There is a nice TED talk, "Building blocks that blink, beep and teach" by Ayah Bdeir introducing the idea of Little Bits  ("Imagine a set of electronics as easy to play with as Legos ..."). Thanks to Cynthia Kinsland for pointing these out to me. Some prices:
- Arduino Coding Kit: 89 USD for 8 Modules / 2 Accessories
- Deluxe Kit: 199 USD for 18 Modules / 3 Accessories
- Smart Home Kit: 249 USD for 14 Modules / 11 Accessories
MaKey MaKey is "An Invention Kit for Everyone" . "Turn everyday objects into touchpads and combine them with the internet." System requirements: "Makey Makey works on Windows, Mac, Chromebook and on many flavors of Linux (Ubuntu, etc.) ... it works on some tablets and mobile devices, but is not officially supported." Cost: 50 USD.
The standard way nowadays to get signals in and out of a computer is by USB cable. What about a cheap USB electronics interfaces? Start by googling "USB kit".
- Wixel Programmable USB Wireless Module . Cost: 21 USD. Some features: 2.4 GHz Radio + 15 user I/O lines, including 6 analog inputs, 2 USARTs, and 7 timer channels. Connecting the Wixel to a computer requires a USB A to mini-B cable, which is not included. I download the manual and scan through it. There is a repository of C code at Github  and a detailed user's guide at the manufacturer's site . You talk to the device through a virtual COM port: "Most of the available Wixel apps implement a USB interface that consists of a single virtual COM (serial) port. This interface allows you to send and receive bytes from the Wixel in the same way you would send and receive bytes from any other serial port on your computer."
- USB Boarduino (Arduino compatible) Kit . Cost: 25 USD. This is a "Solderless Breadboard Arduino Clone".
|||http://www.adafruit.com/product/91 and https://learn.adafruit.com/boarduino-kits|
You can talk to serial ports using C or even python. There is PySerial package available through the Python Package Index ; source code is available at sourceforge and GitHub . On my Mac, running pip search pyserial in a terminal returns pyserial as the first hit. So it looks like you can install pyserial on a Mac using the pip install mechanism. I search the Instructables web site for "python arduino" gives a lot of hits .
I would use an analog multiplier chip to make a (cheap) lock-in amplifier and/or an AM radio demodulator.
- Analog Multipliers and Dividers at DigiKey .
- Balanced modulator/demodulator, AD 630 .
- Low-cost analog multiplier, AD633 .
- "Low-Cost Portable Potentiostat for Biosensing Applications" project by Elliot Friedman (esf59) and Alexander Hartoto (akh75) at Cornell University [link].
- Two open-source electrochemistry-related projects at Public Lab: the Potentiostat project [last updated March 2014; link] and the WheeStat 5.1 projet [last updated December 30, 2013; link].
- “Versatile instrumental assemblage for the study of commercial electrochemical cells", D. González-Arjona, E. Roldán González, G. López-Pérez and M. M. Domínguez Pérez, Chem. Educator, 2012, 17:100-10. doi = 10.1007/s00897122425a (not recognized at http://dx.doi.org) [journal link]. Keywords: Laboratories and Demonstrations; physical chemistry; electrochemistry; laboratory instrumentation; batteries characterization. Not available online.
- "An Improved Galvanostat for the Characterization of Commercial Electrochemical Cells", D. González-Arjona, E. Roldán González, G. López-Pérez, and M. M. Domínguez Pérez, Journal of Laboratory Chemistry Education, 2013, 1(2):11-18. doi = 10.5923/j.jlce.20130102.01 (not recognized at http://dx.doi.org) [journal link]. Keywords: Electrochemistry, Laboratory Instrumentation, Commercial Batteries, Arduino Platform. Available online.