You might be surprised to learn that there's a lot of interest in the Vim mathematics plugin space. Kai Yuan, who has written several interesting Vim plugins (including Join) sent in HowMuch (GitHub: sk1418 / HowMuch). It supports all Visual modes, two output formats (append, replace), and multiple expression evaluators (bc, Python, native).
The documentation has animated gifs so it's easy to see how it works. It even has some spreadsheet-like functionality -- block-wise selection can do sums (something I find hard to communicate without an animation).
I like plugins for plugins. This one is a theme for Airline, called Airlineish (GitHub: paranoida / vim-airlineish, License: MIT) by Rafal Bromirski. It makes it extremely easy to see what mode you're in by using bold colours. Red means replace, and Normal mode is a neutral purple. And Rafal also gets bonus points for using an animated gif in the documentation.
Is there a Tumblr called Vimgifs? Yes there is! http://vimgifs.tumblr.com/