UPDATE: Resources have now been compiled into an official Author Resources KnowledgeBase article. Feel free to add new suggestions here to this unofficial resources list.
This thread details essential third party resources for authors. Mostly it's free / open source / creative commons. There's one or two very popular / essential pay resources too.
Maybe this thread will become part of the Wiki one day... let's see. In the meantime, can you think of some essential resources?
Below are the collected highlights from the rest of this thread....
Audiojungle - Videohive's sister site. With permission from the author, use watermarked audio in your previews for free.
Audio Jungle Radio - The Audio Jungle Radio plays Audiojungle collections or search results as a playlist in the background, so you can continue to work while you search for that perfect track!
Font Squirrel - Only the best commercial use free fonts
Lost Type - A nice collection of donationware fonts
Dafont.com - Thousands and thousands of free fonts of varying quality.
The League of Moveable Type - Several excellent, high quality typefaces
Font Fabric - Several excellent, high quality typefaces
MG Open - Videohive typeface
Fontshop Free Fonts
http://www.fontshop.com/fontlist/staff_picks/free_fonts/ - quality typefaces from otherwise commercial font reseller.
Fonts Installed as Part of CS5
Which typefaces you can use, firm in the knowledge that your customers will have them too.
Fonts Installed with Windows 7
Which typefaces you can use, firm in the knowledge that Windows 7 users will have them too.
Fonts Installed with Mac OSX
Which typefaces you can use, firm in the knowledget that Mac OSX users will have them too. Also included is information about crossover with Windows installed fonts.
Expression universalizer - Make your After Expressions work in all language versions. Name your own price
BG Renderer - Render in the background while you continue to work in After Effects (and more). Indispensible. $14.99 basic / $34.99 pro - free trial
Open Sesame - Backwards compatibility and more for After Effects. $49.99
iExpressions - Amazing expression stuff made easy. Modular - bundles available from $4.99 to $44.99
True Comp Duplicator - Duplicates the entire composition hierarchy. Name your own price.
3D Extruder - Automatically creates 3D extrusions with bevelling, shading and reflection. $34.99
Color Scheme Designer - The very useful tool for every designer
Adobe Kuler - Color schemes
Photodune - Duh! Bit of a no brainer, this one. Photodune is a sister site to Videohive and although you need to pay a small amount to licence photos, the licensing deals are great for inclusion in previews.
Morguefile - Masses of free to use photos.
Wikimedia Commons - Creative commons and some classic out of copyright images.
Public Domain Pictures - pretty average free stock site. Actually, I'm sure this one can be replaced with something better. And anyway the licensing is pretty dubious.
bigfoto.com - Use their photos. The only condition - a link back to their site.
David Niblack - Use their photos. The only condition - a link back to their site.
http://www.sxc.hu/ - Free stock photo site
Blender Open Movie Project - Various great animated pieces created in Blender
Open Footage - Excellent timelapses / HDRI maps / panoramas
stockfootageforfree - If you need footages for preview in your project, here is a nice site offering couple of free stock videos.
Vimeo Free HD
"Vimeo Free HD":http://vimeo.com/groups/freehd/videos - Free HD clips. Some are a bit shaky but they're free.
X Stock Video - Some nice HD footage for free and check the LINKS-tab for some more.
Digital Juice - VideoTraxx HD - The most frequently used video for previews on Videohive. Set1 $249. Set 2 $169.
http://mayang.com/textures/ - Masses of free textures
http://urbandirty.com/ - Grunge textures
Lost and Taken
http://lostandtaken.com/ - Grunge textures
http://www.cgtextures.com/ - 3D textures
40 Free vector and Photoshop brushes - Free vectors and PS brushes.
Adobe After Effects - Obviously!
Cinema 4D - 3D everything
Adobe Creative Cloud - A new way to licence Adobe software
After Effects trial - 1 month free trial version
Cinema 4D demo - 42 day free trial version
Blender - Free 3D software
Cinema 4D plug-in for After Effects - Import C4D .aec format into After Effects
Trapcode Particular - fast and effective pseudo-3D particle generator $399
Trapcode Form - fast and effective thingummy for making kind of... well... Trapcode Formy stuff. $199
Video Copilot Optical Flares - highly controllable lens flare plug-in $125
Reelsmart Motion Blur - Must have tool for post-application of motion blur to 3D and other renders. $89.95 / $149.95 (pro version)
Videohive / Envato
Brian's uploading 101 video for new authors....
Free File of the Month
"Submit your file for Free File of the Month here!":http://www.formspring.com/forms/?618060-pwoYpvH12g (note: you only need to upload it once. After that it remains in a holding bank for the future.)
What resources you can and can't include in your files
"wiki videohive basics":http://wiki.envato.com/tag/videohive-basics/
"Envato logo - vector":http://sharealogo.com/internet/envato-vector-logo-download/
"Envato logo - PNG":http://img220.imageshack.us/img220/3766/envatologo.png
"Envato logo with text - PNG":http://img263.imageshack.us/img263/4770/envatok.png
All market place logos
"Download zip":http://envato.s3.amazonaws.com/wiki/EnvatoMarketplace_Logos.zip - almost all market place logos in PSD and Vector EPS versions: graphicriver, videohive, themeforest, activeden, audioungle, 3docean, codecanyon and envato.
Motion Graphic Design Census 2010 Results - An extremely useful census of work practices and rates in the motion graphic design industry.
Region of Interest - Todd Kopriva's Blog about what's current in the world of After Effects.
Todds' part of the After Effects technical support team at Adobe, foucssing on "one-to-many" support. In other words, helping people out on various blogs and forums. You'll even catch him on the Videohive forums from time to time!
After Effects Feature Request / Bug Report - Page where you can log feature requests and bugs. The feature requests all get read, although you probably won't hear directly back about it. This is how you get involved in the development of your favourite program.
Add your own in a reply to this thread.... other users have submitted loads more great links. By the way, I'm formatting all the posts in a certain way, for ease of reference.
The best links will be copied into this post.