Halo 3's in-engine replay system was the high water mark of gaming for me.
Ah, the good old days of watching live competition of quake through the game itself, chatting with others basically through the game console.
Pretty cool system.
The game engine, Source, is also using client-server architecture
I'm sure the technology still exists in the engine, but it's no longer the key feature it once was. HLTV/GOTV was launched with some fanfare back in the day.