I remember reading something a while ago about a 'law' of software
development, which said that that all software will expand until it
ultimately has the capability to send email. I can't remember where I
found this, though, and what the name of the law is! Any help would be
appreciated :)

I believe you're thinking of Zawinski's Law of Software Envelopment.

"Zawinski's Law of Software Envelopment (also known as Zawinski's Law)
relates the pressure of popularity to the phenomenon of software
bloat:

Every program attempts to expand until it can read mail. Those
programs which cannot so expand are replaced by ones which can.

Examples of the law in action include Emacs and Mozilla.

This law is attributed to Jamie Zawinski, who popularized it. It may
have been inspired by the humorous Law of Software Development and
Envelopment at MIT, which was posted on Usenet in 1989 by Greg
Kuperberg, who wrote:

Every program in development at MIT expands until it can read mail."

Wikipedia: Zawinski's law of software envelopment
http://en.wikipedia.org/wiki/Zawinski's_law_of_software_envelopment

My Google search strategy:

Google Web Search: "zawinski's law"
://www.google.com/search?hl=en&q=%22zawinski%27s+law%22

I hope this is precisely what you need. If it is not, or if anything
is unclear or incomplete, please request clarification; I'll be glad
to offer further assistance before you rate my answer.

Best regards,
pinkfreud

Awesome! Exactly what I wanted, and a lot more.

Thank you very much for the five stars and the nice tip!

~pinkfreud