A popular MPEG4 derived video CoDec.
Originally released in 2000 as "DivX ;-)", the codec started life as a hacked version of MicrosoftCorporation's MPEG4 codec, that could save to AVI instead of just Microsoft's ASF, and had support for MP3 audio.
In 2001, the codec was cleaned up and proprietary parts rewritten, and it was released as "DivX" (without the smiley.) An open source project called Project Mayo emerged around it, and it was later forked in two directions, a divx.com commercial enterprise, releasing proprietary "pro" versions with more encoding features as well as a free player, and OpenDivX, Free software from Project Mayo. OpenDivX was the basis for the newer codec Xvid.