Tools: Frink calculator

I highly recommend using Frink instead of, say, windows Calc. It's a free, but not open source, Java program. It's command line driven but very powerful. I especially like the built in conversion capabilities and translation features. Here's the feature list:

• Tracks units of measure (feet, meters, tons, dollars, watts, etc.) through all calculations and allows you to add, subtract, multiply, and divide them effortlessly, and makes sure the answer comes out correct, even if you mix units like gallons and liters.
• Arbitrary-precision math, including huge integers and floating-point numbers, rational numbers (that is, fractions like 1/3 are kept without loss of precision,) and complex numbers.
• Advanced mathematical functions including trigonometric functions (even for complex numbers,) factoring and primality testing, and base conversions.
• Unit Conversion between thousands of unit types with a huge built-in data file.
• Date/time math (add offsets to dates, find out intervals between times,) timezone conversions, and user-modifiable date formats.
• Translates between several human languages, including English, French, German, Spanish, Portuguese, Dutch, Korean, Japanese, Russian, Chinese, Swedish, and Arabic.
• Calculates historical buying power of the U.S. dollar and British pound.
• Calculates exchange rates between most of the world's currencies.
• Powerful Perl-like regular expression capabilities and text processing.
• Supports Unicode throughout, allowing processing of almost all of the world's languages.
• Reads HTTP and FTP-based URLs as easily as reading local files, allowing fetching of live web-based data.
• Runs on most major operating systems (anything with Java 1.1 or later,) as an applet, through a web-based interface, on a wireless Palm VII, on an HDML- or WML-based webphone, and on many mobile phones and hand-held devices.
• Installs itself on your system in seconds using Java Web Start and automatically keeps itself updated when new versions of Frink are released.
• Runs with a Graphical User Interface (both Swing and AWT) or a command-line interface.
• User interface has a Programming Mode which allows you to write, edit, save, and run extremely powerful programs even on a handheld device.
• Powers Frink Server Pages, a system for providing dynamic web pages powered by Frink.
• Frink is a full-fledged programming language with arrays, dictionaries, functions, loops, even object-oriented programming and self-evaluation.
• Frink allows Object-Oriented Programming, which allows you to create complex data structures that are still easy to use.
• Java Introspection layer allows you to call any Java code from within Frink.
• Frink can also be embedded in a Java program, giving your Java programs all the power of Frink.
• Did I mention it's free? If you find it useful, please donate something. I'd really appreciate it!

Here are some examples that I've used:
// Convert from US Dollar to Brazilian Real
USD -> BRL
2.5992524549939437418

// Calculate how many hours I worked today
// left work at 7pm arrived 8:30am and had an hour lunch
#19:00# - #09:30# - 1 hour -> hours
17/2 (exactly 8.5)

// I need five cups of sugar, how many kilos of sugar do I need?
sugar * 5 cups -> kg
1.0

The docs are fun to read, and gives lots of interesting examples. Even the units file is an interesting read.
Highly recomended.