Orbiter doesn't include any built-in sound features, but thanks to DanSteph, I would guess that few "Orbinauts" ever think of this fact. Dan's OrbiterSound 3.0 has to be downloaded and installed separately (a painless process), but once it's installed, it is so smoothly integrated with Orbiter, that it's hard to imagine that it's "just" an add-on. It is probably the one truly "must have" add-on for Orbiter - it adds immensely to the experience to hear the perfectly synchronized sounds of mechanical systems, thrusters, and engines (only when inside the spacecraft, unless you are in the atmosphere, when external sounds can be heard, growing fainter as you move away from the sound source). Add-on developers can provide custom sounds and "radio chatter," and it even supports MP3 music, providing a cinematic soundtrack when you are in external views (one of many ways to configure OrbiterSound).
Dan's other widely-used Orbiter add-on is the cool DeltaGliderIII (a version IV is now in beta), which adds to the challenges of flying the basic DG by making you worry about keeping your passengers alive (don't subject them to super-high G forces, extreme temperatures, or lack of oxygen). Dan also developed the professional Microsoft Flight Simulator add-on FsPassengers (Orbiter seems to be his hobby, luckily for us).
Dan's Orbiter Page also hosts a forum (in both English and French) as well as Orbiter-related news items and some feature articles. In addition to all this, he also provides the cool Orbinauts World Map which shows the location and some information about the 612 people who have so far registered as "Orbinauts" (you can find someone called "FlyingSinger" near Boston). The screen capture here shows Europe, with "DanSteph" and "Martins" (Orbiter's creator) highlighted. Dan has contributed so much to la communauté d'Orbiter - merci, Dan!