The Official Sites
First, Geogebra.org
 Click on Downloads for the free mobile or computer Pick your OS to download to your machine  including tablets! Tablets are at the top, laptop/desktop below. (Note: Windows 8 can download either: get the desktop.)
 Click on Materials to browse GeoGebraTube to see what people have made already. These sketches can be run in a browser on desktop or mobile. The uploaded materials are searchable, like YouTube. Worksheets can be downloaded (everything is CC3.0), liked, or organized into GeoGebraBooks.
 Click on Start GeoGebra to open Web GeoGebra in your browser, which has most of the functionality of the full program.
The Help tab takes you to the wiki, which has a Manual, the Forum (I've never waited more than a day for help), Tutorials, and Tool Tips and Tricks. Here's a Quick Start guide (pdf).
For Twitter Math Camp 2015 Jed, Audrey and I pulled together a good list (Gdoc) of resources and tasks for GGB users at multiple levels.
On Twitter you can include #ggbchat or @geogebra to get help. But a lot of people in the #MTBoS can help or connect you with someone who can. The Geogebra Facebook page is a good way to keep up on developments, meetings, etc. There is also an active GeoGebra community on Google Plus. There is an online, peerreviewed journal, North American GeoGebra Journal. I maintain a twitter list of excellent GeoGebra folk, also.
GeoGebra Resources by other people
 NEW: Tim Brzezinski's GGBbook with a dynamic illustration of every CCSS geometry standard. Wicked impressive.
 Jim Doherty's free geometry textbook, loaded with GeoGebra support and activities.
 @bmk2k's blog for GeoGebrArt.
 Jennifer Silverman's GeoGebrarich geometry posts.
 Audrey McLaren's great GeoGebra Voicethread intro, and her GGB page
 Great collection of professional development sketches by Judah Schwartz, a mostly retired math educator from Harvard. He did foundational work on Mathematical Habits of Mind. (Dan Chazan was one of his students.) They're on the 'Tube, too, so adaptable and usable for inspiration. https://sites.google.com/site/mathmindhabits/
 GeoGebra Institute of Iowa's K8 activity collection.
 Our Grand Valley GGBGV site has our Intro to GeoGebraTube and Intro to GeoGebra workshops.
 Guillermo Bautista's GeoGebra series.
 Irina Boyadzhiev's applets for college math.
 Broken Airplane's GeoGebra resources
 Tim Cieploski's GeoGebra sketches for HS math.
 Tony Cron's GeoGebra for remediation math
 Bowman Dickson's GeoGebra for the Classroom tutorials and links
 Linda Fahlberg's Geogebra Wiki (for teachers)
 Mike May's excellent GGB 4.0 Workshop
 Marc Renault's Tutorials and his GeoGebra samples  in particular see his beautiful Calculus Applets
 Richard Wade's nice intro tutorial: Intro GeoGebraNick Bennett's math wiki; more of a focus on teaching the students to use GeoGebra than complex sketches.
 Nordin Zuber's GeoGebra HowToTerry Lindy's collection of GeoGebra applets for IMP. (wide variety of HS math)
 Brevard School's GeoGebra page
 GeoGebra Math
 GGB Wiki Tutorials
My GeoGebra Work
All Blog Posts, usually discuss use with students or GeoGebra methods.All Tumblr Posts, usually just the sketch with a picture & links, the bulk of my GeoGebra sketches.
All GeoGebraTube uploads (my profile) as well as some collections of others' sketches.
Number
Modeling:
 Calculus:

GeoGebraTube Profiles to browse
 Daniel Mentrard  the One
 Anthony Or  amazing
 Jennifer Silverman
 Professor Ortiz
 Michael Borcherds
 Steve Phelps
 Ryan Hirst
 Dan Pearcy
 David Cox
 Tim Cieplowski
 Michael IROIR
Anything is fair game!