Don't have a 3DO? Well, don't worry -- there's demos enough to go around. The history of playable Wing Commander demos is rich and interesting.
The first was a 1994 demo of Wing Commander III (here - 29 megs), initially released on the first ever PC Gamer cover CD. Demand was very high - PC Gamer ran out of CDs quickly and the demo was very rare for several years. The demo is a unique patrol mission, which plays slightly differently than the finished game.
Several versions of the Wing Commander IV Demo exist. The first included a cinematic trailer and was released on magazine cover discs (here - 43 megs). That was a huge file for an era when ordinary people didn't have CD burners or fast internet connections... so Origin later released a smaller internet version with the trailer removed (here - 14 megs). Even that seemed like an impossible download at the time, though! The mission is a fun gauntlet where you have to shoot down as many pirate fighters (and frigates) in a set time limit. There's a neat easter egg, too -- lock onto Dekker's shuttle for a few seconds to recieve a special weapon. A Macintosh version was later released (here - 6 megs).
Then came a little known demo - a playable preview of Privateer 2 (here - 54 megs). The Privateer 2 demo was never released online by Origin - it only appeared on a magazine cover disc. The demo does not have a new mission... instead of lets you explore the entire game universe for a limited amount of time. It's most interesting because it contains two short video clips not included in the original game, greeting Joe the Bartender.
Wing Commander Prophecy took demos even further -- with two different demos and a total of six unique missions! Origin initially released a '3Dfx Test' demo (here - 13 megs) to let users who had 3Dfx cards see what the game was going to look like. The 3Dfx Test is a timed, scored gauntlet, which lead to fierce competition among fans lucky enough to have the special graphics cards at the time. Then, an entire series of missions was made available to everyone. The Wing Commander Prophecy Demo (here - 38 megs) contains an entire four mission series which takes place 'between' other missions in the game. The demo even has a winning and losing track! A separate pack of FMV movies (here - 19 megs) which could be added to the demo was also posted.
Finally, Wing Commander: Secret Ops (here - 112 megs) is itself a demo. While the later episodes were not kept available for download, the first remained online to serve as a playable demo for Wing Commander Prophecy Gold.