Here is a list of some interesting projects I have and still work on. There is no particular order, all of them are Open Source.
vcs_query is a python program that can be used to query eMail-Addresses
from vCards and either use them directly, or use them as input for
console-based programs like mutt or Vim.
This lets you interface your favorite eMail MUA, like mutt, with a common way to store contact data, rather than using the limited on-board ways of it. For Vim you can use a simple user-defined completion (example is on the linked page) to auto-complete addresses in files while you edit them (again, while writing a message for mutt to send, or for example, in git commit messages and patch files).