ChatQalc

An interface to Qalculate! qalc - the best cli calculator. (Included for Windows/Linux)

Installs and runs portable interactive qalc cli calculator when you start Minecraft
Operates in chat screen (T or /)
NumPadEnter or Ctrl+Enter = Send message to qalc instead of chat
Ctrl+Tab = Convert last expression without spaces to calculated result (escape spaces with \)
Shift+Enter = Send message to qalc and replace chat field with last line of the result (You can look at the result and decide to share it with everybody by pressing Enter)
Ctrl+Shift+Enter = Same as Ctrl+Enter, but result is broadcast from your name to the whole server. And without colors. (Use when you want to share multiple lines)
Shift+Tab = Get completions
Chat history of your queries. Very helpful.
Open config in ModMenu to get a graphical interface. All settings are there. (Edit tab)
Some Minecraft-related units and variables (100it = 1 stack + 36 items) it st sb pdc pdcwh gt iph hs, ch|chunk|chunks

Most notably:

/help then Ctrl+Enter
Use -> for unit conversion (1km->m = 1000 m)
Fault-tolerant parsing of strings (almost natural language)
Ability to disable functions, variables, units or unknown variables for less confusion: e.g. when you do not want (a+b)^2 to mean (are+barn)^2 but ("a"+"b")^2
Verbose error messages, Arbitrary precision
Symbolic calculation
Functions, Units, Variables, Constants (persistent between restarts)
Gnuplot (opens a new window with plot, you have to install GNUplot separately for it to work)
Comments start with #

You can get a 30KB noinstall version if you install or link Qalculate to ./minecraft/config/chatqalc/ manually or run the full version once.