There are many sites created specifically to host items created by people for hobby and related purposes. Listed below are ones I have come across worth mentioning.
These websites are entirely seperate from mine and have no association. Do not contact me about them.
0 A.D. - A full-fledged game being made by a group of RTS fans. The game has been in works for several years and looks promising.
Albino Blacksheep - Big site hosting media such as videos, audio, and notably flash animations and games.
Audiodraft - Contest hosting site for music. Enter contests created by various companies or create a contest yourself for your projects that need music.
Denny Schneidemesser - Used to be known as danman87 on Newgrounds around 2009 for a bit before pulling out to produce music commercially. Pretty inspiring stuff.
Heavengames.com - Collection of gaming fansites with strong design communities (mods, user-created content), namely for RTS games like Age of Empires.
HotScripts.com - Relatively large site and community that shares scripts people have made for web designing.
K2xL - Site of a flash artist, K2xL or Danny Miller, who created several popular flash games such as Stick Avalanche and Psychopath.
The Legendary Lillypad - Site of a flash artist, LegendaryFrog, who has created many popular flash animations (pretty funny ones too ^_^ ).
Machinima.com - Big site based on the creation of movies and shows via recording from games, a technique also known as Machinima.
Newgrounds.com - Big site featuring mainly flash work and music by countless artists. There are both amateur works and more professional works as basically anyone can submit. You just have to dig. There's also an Interview Index so you can get an insight to how a lot of the artists get where they are today.
OCRemix.org - Similar to the Machinima and Newgrounds exceptly solely focused on remixing video game music. The quality here is much higher because submissions (at least on the main parts of the site) are not open to everyone, but it's great for finding quality music others have made, as well as reading up on tutorials and getting help.
Rooster Teeth - Group based on creating movies and cinema from games. I found them out through their "Red vs. Blue" series at Machinima.com, which has reached over 70 episodes last time I checked.
Sound Cloud - Site for sharing sounds and music you create, similar to Newgrounds but strictly for audio.
Symphony of Spectars - Handful of Newgrounds artists (initially MaestroRage and a couple others) who banded together to produce music for both large corporations and small groups. Interview with some of the artists here.
The Vandhaal Website - A site featuring mainly real-time strategy game designs and media by The Vandhaal. His site also hosts numerous other creations, mainly movies and cinematics, by various authors in a section called "The Theatre of the Mind."
UGO Player - Big site hosting media such as music and notably flash animations and games.
XGen Studios - Site of XGen Studios, a group which has developed a number of very popular flash games.
Xstormer1 - Music artist who makes very good music, generally Techno/Trance, and has started his first professional album. One of his most noteworthy songs is Flames of the Sky, which had a sequal (Flames of the Sky II) two years later.