libmls is a library that can be used to measure the frequency response of a loudspeaker or a room using your computer and an ordinary soundcard. It can perform both MLS (Maximum Length Sequence) and Sinus sweep measurements. It supports ALSA, OSS, and the sound system used in Windows.
Electronic Engineering Tool is a Web-based tool that includes an electronic formula calculator and converter functions. It very useful when working with electronics, both for engineers and amateurs. For example, it includes converters for dBm-to-Watt and Fahrenheit-to-Celsius, and it can calculate Ohm's law, filters, thermal resistant C/W, SWR, coil inductance, capacitor capacity, and more.
Balsa is an asynchronous circuit simulation and synthesis system. Balsa is built around the Handshake Circuits methology, and it can generate gate level netlists from high-level descriptions in the Balsa language. Both dual-rail (QDI) and single-rail (bundled data) circuits can be generated.
WREC controls an SCPI-programmable power supply (such as Agilent E3631A) to reform large aluminum electrolytic capacitors. It was written specifically for the Computer History Museum PDP-1 Restoration Project, but should be useful for reforming capacitors for almost any old equipment, even audio gear.