There as been a lot of discussion in the comments section of this question. I can gracefully accept the fact that it isn't well-formed as it currently stands, but I do want the question to remain on the site as a useful resource for those who wish to extend Emacs, just as the TeX.SX question it mirrors does.
Perhaps the format of the answers needs to be changed; perhaps the question needs to be made more specific (though I don't know what words could be provided that would give more detail); perhaps the scope needs to be broadened; … I'm very open to suggestions
I'll note that broadening the question to include development tools for all languages would effectively nullify the question as essentially all packages have direct use in the development cycle. That is a question I would most certainly close as 'Too Broad'.