Woa, I didn't expected such enthusiasm...
Really, it's the best chapter in HL2. Maybe just "Follow Freeman!" is a little bit better, because of the teamplay and stuff? Both city levels ("Anticitizen 1" & "Follow Freeman!") are really reminding me of the defense on Stalingrad.
Truely said, pretty much all levels are based on some other event or game:
"We don't go to Ravenholm" - Resident Evil
end of "Sandtraps" (where you aquire the Pheropod) - D-Day from the Second World War (all these machinegun nests and beach assoult and stuff...)
"Nova Prospekt" & "Entanglement" - Alcatraz prison (atleast in any movie where it can be seen)
end of "Nova Prospekt" (where your way gets cut off by the mechanical walls) - not sure where I've seen such huge machinery, but it reminds me of UFO:Aftermath in a way (just like the moving walls, the biomass crawls forward, overtaking new regions)
"Anticitizen 1" & "Follow Freeman" - already said it - Stalingrad's defense
"Dark Energy" & "Our Benefactors" - UFO series, the aliens coming from a portal and trying to enslave the human race. The scenery from the Citadel looks like the last mission from Aftermath
...and all these areas, where you have to avoid sniper fire - "Enemy At The Gates"
And if I tell ya that "Route Canal" and "Water Hazard" are based on my home city and surrounding areas you're not going to believe me...but wait to see the pictures!