It's only happened a few times but watching my wife give our friend a dedicated blow job is damn hot.
They're both naked after a hot make out session. He's on his back, she's laying on his leg sucking his dick. He always tells her he's getting close, she usually pulls off and they move on to something else. A few times she just kept at it, his body got stiff, he grunted and she pumped his dick and swallowed all of it. The hottest thing to watch ever.