What are good tools for Emacs package development?

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'.}