Reference to Guice.

GMaps - my ex-TA was an intern at Maps team, she told me that mostly it is written in C++. She also told me how surprised she was when she found out that most big products were written in C++/Java instead of Python.

GMail - might want to ask Paul Bucheit, but I remember vaguely coming across a blog post/article saying that it is written in C++ and Java.

Joshua Bloch (Effective Java author) said that they developed in-house middleware that bridge Google's infrastructure with Java (BigTable, MapReduce were written in C++).

I suspect Google is using JNI to access C/C++ based framework/infrastructure.

So far, I only know 2 products written in Python:

1) YouTube (purchased)

2) Google Groups (semi purchased I think, from the people that brought you ClearSilver template in Python).

I heard YouTube had to integrate their code to Google's infrastructure, but I don't know if they had to re-write some stuff.