Which SC2 units can accidentally cause friendly fire?

The only units that actually cause friendly fire from splash damage are:

• Siege tank shells
• Psionic storms
• Widow Mine attacks (except for other widow mines)
• Raven's Seeker Missiles
• Ghost's EMP
• Ghost's Tactical Nuke

Some units that you might expect to cause FF splash damage but in fact don't are:

• Yamato cannon on BCs (doesn't cause any splash)
• Planetary Fortresses
• Hellion/Hellbat attacks
• Thor attacks
• Baneling explosions
• Infestor's fungal growth
• Ultralisk attacks
• Mutalisk attacks
• Colossi attacks
• Archon attacks

Edit: See this website for more details.

Currently the units which can cause friendly fire are Siege Tanks (in siege mode), Psionic Storms, Widow Mines, and Raven's Seeker Missiles.