From Valve Developer Community

This is a list of links to external sites dealing with creating Mods with the Source Engine.

File Hosting Sites

There are a wide variety of options that you can utilize to make your custom maps and mods available to the masses. You can upload them to sites dedicated to gaming, get a server to place it in their rotation, or you can host it yourself.

This is a list of sites where you can upload content to.

General hosting

http://www.home4files.co.il: Limit: 500MB/file; No account needed; Upload and download unlimited files for free; Files hosted Forever until user delete it.
http://www.uploadpedia.com: Limit: 100MB/file :No account needed; Upload up to 10 files at once; Upload and download unlimited files for free
http://www.BOXSTr.com: Limit: 10GB; Premium account offered; Desktop and web uploading; 250Mb File Size; 10GB Bandwidth; Hotlinking
http://www.sendspace.com: Limit: 300MB; Premium account offered; File Tracking & Statistics; Files deleted after 9 days with no download
http://www.4shared.com: Limit: 50MB; Premium account offered
http://www.axifile.com: Limit: 150MB; FTP capability
http://www.esnips.com/: Limit: 5GB
http://www.fastshare.org: Limit: 50MB/email webmaster@fastshare.org to host files up to 4GB
http://www.filedropper.com/: Limit: 5GB; Premium account offered, with 50GB.
http://www.filesend.net: Limit: 120MB
http://www.filevip.com: Limit: 100MB
http://www.gigeshare.com: Limit: 2000MB; Expires: 90 days inactive
http://www.keepmyfile.com: Limit: 60MB; Expires: 40 days inactive
http://www.mediafire.com: Limit: 100MB; Expires: Never
http://www.megaupload.com: Limit: 250GB; File Size Limit: 500MB; Expires: 90 days inactive; Premium account offered
http://www.ohshare.com: Limit: 80MB; Expires: 30 days inactive
http://www.quicksharing.com: Limit: 500MB; Expires: 30 days inactive
http://www.rapidshare.com: Limit: 100MB; Expires: 90 days inactive; Premium account offered
http://www.savefile.com: Limit: 60MB; Expires: 30 days inactive
http://www.sendmefile.com: Limit: 50MB; Expires: 28 days inactive; Premium account offered
http://www.storeandserve.com: Limit: 100MB; Expires: 30 days inactive; Premium account offered
http://www.turboupload.com: Limit: 70MB
http://www.ultrashare.net: Limit: 100MB; Expires: 21 days inactive; Premium account offered
http://www.uploaded.to: Limit: 250MB
http://www.uploading.com: Limit: 300MB; Expires: 30 days since upload if there are no downloads
http://www.yousendit.com: Limit: 1024MB; Expires: 7 days; Premium account offered
http://www.zupload.com: Limit: 500MB; Expires: 30 days inactive
http://www.badongo.com/: Inactive files expire after 50 days for guests, 90 days for registered members; Bandwidth: 4.8GB/day for guests, 12GB for registered members; Premium account offered

General gaming hosting

These hosts only host for gaming related content, and not just HL2-based games.

http://www.atomicgamer.com/uploads.php: Membership required
http://www.filecloud.com: Membership required
http://www.filefront.com
http://www.fileplanet.com: Membership required
http://www.fileshack.com: Files must be moderator approved before made public
http://www.fpsbanana.com: Membership required
http://www.2manydownloads.com: Files must be moderator approved before made public
http://www.moddb.com: Membership required
http://www.tf2maps.net: TF2-related content only

HL2 & mods related hosting

These websites are only used for HL2-related files.

http://halflife2.filefront.com/: Also hosting files for other Valve games [1], membership required

Image Hosting Sites

This is an incomplete list of websites that offer free image hosting, which may be of use to mod makers and mappers looking to promote their work.

PicMember.com

Allows hosting of the full-quality version if you want, also has various thumbnails that can be embedded or downloaded, 4GB starting storage. No upload limit