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.
Nyagua is a little piece of software that may help you to manage your home aquarium. It can store data about measurements in aquariums and produce graphs from these. It can store data about expenses and produce a report with yearly, monthly, and daily costs. It reports the compatibility between fishes and plants in its database. It can import fishbase.org XML data, and your own pictures can be added.