Jobs and Networking

80Twenty - Job postings for creative careers.
Angel List - Sounds like an exclusive club but anyone can just sign up and create a profile for themselves and their product/company.
EventBrite - Generally larger, more well-organized local networking events often by companies.
GlassDoor.com - Reviews and other data on companies for deciding whether you want to work there or not.
GoFundMe - Fundraising site for projects both business and personal.
IFTTT - "If-this-then-that".  Useful service for reading from one site/feed and copying content to another when managing multiple online presences.
IndieGogo - Fundraising site for projects, products, and whatever else you want to crowdsource.
InternMatch - Internship postings in somewhat of a blog format, very sortable.
KickStarter - Fundraising site for projects, products, and whatever else you want to crowdsource.
LinkedIn - Basically a career-oriented version of Facebook with some blend of networking for interests/projects.
Meetup - Find local networking groups for interests but you tend to get more dabblers than serious committers.
NextDoor - Somewhat of a dashboard/bulletin for people living in the same neighborhood, but they have to be checking this site to see obviously.
Patreon - A more perpetual funding style where fans can commit to keep chipping in some amount for every project you release.
StackOverflow Jobs Board - Primarily tech-related jobs.
WorkFrom.co - Listings of open work areas (coffee shops, etc) in the city.
Velvet Jobs - Jobs board for entertainment, fashion, and finance.
YCombinator Jobs Board - Jobs board for startups that came out of YCombinator, often small founding teams that are well-funded and flat organizationally.
YouTube - The medium is technically video, but it's pretty much the place to be to build up exposure for just about any endeavor.

CreativeCow - Generally strong discussion board for most things on the video production side.
Production Hub - Listings of film-related jobs.
Stage32 - Social networking site built around film and related projects.
Vimeo Wanted and Offered - Forum for people doing video-related work to request and offer help.

Behance - Portfolio site created by Adobe.  Not sure how effective it is, though the art schools seem to be all over it.
DeviantArt - Pretty strong artist community but tends to tilt towards fanart or anime more now.

GameDev.net - Community for game development, which often needs music.
IndieDB Jobs Board - Job postings relating to Indie games and game dev comunity, often needing music.
IndieGamer - Community for general indie-game making, which often needs music scoring.
NewGrounds - Slowly fading away on the music side, but I've met many talented composers from here.  There's also more of a community feel with discussion boards and collaborations across media.
RPGMaker - Community based around making RPG-genre games, but there's often a fair amount of a demand for scoring/music as well.
SoundCloud - Less of a community and more a portfolio/subscriber site to advertise works, but it's now the dominant place for indie musicians/composers.
Unity 3D Forums - Forum for unity game developers.  Many projects looking for music potentially.
Vimeo Wanted and Offered - Forum for people doing video-related work to request and offer help; many times it relates to music composing as well.

GameDev.net - Community for game development.
Gamasutra Jobs Board - Games-specific jobs board on the game developers site.
IndieDB Jobs Board - Job postings relating to Indie games and game dev comunity,, both paid and unpaid.
IndieGamer - Community for general indie-game making.
NewGrounds - Slowly fading away in general, but there's still a fair amount of quality Flash games that come out. There's also more of a community feel with discussion boards and collaborations compared to most sites.
RPGMaker - Community based around making RPG-genre games.
Unity 3D Forums - Forum for unity developers.  Many projects looking for collaborators, etc.