As Halo's OG composer, Martin O'Donnell enjoys posting some seriously nostalgic footage to his YouTube channel now and then, both related to his music and the Halo universe at large. In that vein, he made a video on "Halo The Movies", a DVD of all cinematics made until 2002, including footage from before the official announcement of Halo: Combat Evolved in 1999.
In the video, O'Donnell focuses on some of the rarer clips, like a fun teaser shown at the Fanfest convention in Chicago all the way back in 2001, or an reaction-style video called "Evolution Of Halo" in which a few of its devs react to what was we once thought of the pinnacle of PC game graphics. There is also a spoof of the Budweiser Wazzup ad, acted out within the game. Remember that ad? It's from 1999, approximately one billion years ago. Halo was on the meme train back when I was still on dial-up.
O'Donnell points out most of the clips on the DVD have probably made their way online at some point, so this fun look back may encourage you to look for some old footage and enjoy a bit of good old-fashioned nostalgia. Speaking of which, you can play Halo 3: ODST, which released as part of the Master Chief Collection this week.