Half-Life 2: Episode One Spoilers
Vortigaunts
The introduction shows vortigaunts using their powers to stop the G-Man from putting Gordon Freeman in stasis and saving Alyx Vance and Gordon.
Zombine
Zombines are Alyx's name for Combine zombies.
Sometimes they do a kamikaze attack by grabbing a live grenade and running towards you.
The Citadel
The Citadel will explode at the end of Episode One but it seems that isn't the direct result of the actions of Freeman. It seems that the energyreactor of the Citadel was unstable and so deactivated all Combine teleports and communication-devices. The explosion of the Citadel was meant to send a message to the Combine. The content of this message is unknown but luckily Alyx managed to make a copy of it. It is clear that the Combine are trying to take control again, after Breen was killed.