U - L - T - R - A r e a l i t y . info
Other places for Ultra 64 knowledge
ultra64.ca (HTTPS?) -- Official SDK downloads for the Ultra 64.
Nintendo 64 forever (HTTPS) -- Ultra 64 forum. Lots of friendly and knowledgeable people, recommended visit if you're on a TLS-compatible web browser. It uses the Tapatalk backend, I'd love to set up an acmlmboards alternative instead as I dislike the Tapatalk forum software quite a lot.
Official Ultra 64 programmers' manual -- Can't get much of a better source of knowledge than Nintendo's own formerly-confidential programming manual. It's HTTP, thankfully -- an indispensable (yet underappreciated) attribute. If that ever changes, expect this link to change to a locally hosted copy.
n64brew Wiki (HTTPS) -- The motherload of all Ultra 64 knowledge, and an indispensable tool in creating this website, but HTTPS only and as such not universally accessable -- especially not as an on-line manual for systems running the Official SDK on Windows 9x/NT 3.51/NT 4.0/IRIX.
n64brew Server (Discord) -- If you can't find it there, you can ask those that authored it. They're a nice group of people, but this option demands that you sign up for the proprietary and freedom-disrespecting Discord service. Not to mention, the archives aren't publicly searchable, so there's a ton of knowledge just locked away on there.
n64squid (HTTPS) -- A fellow Ultra 64 programmers' website and news host. I like their site. It's not what I'd exactly consider lightweight or responsive, though.
Other places for general programming knowledge
learn-c.org (HTTPS) -- an interactive C programming tutorial.
Basic Programming Concepts - BYU Office of Digital Humanities (HTTPS?) -- exactly what it says it is.
LLVM -- The LLVM compiler suite, one option for C compilation.