Do you know of well designed open source applications that are instructive to analyse?
Specifically, I'm interested in practical applications object-relational mapping in C++ based programs, where there is a good separation between a domain model and persistence/serialization functionality.
If you include well designed open source libraries that are instructive to analyse, have a look at Boost.
About persistence / serialization (not relational mapping though), have a look at Boost Serialization, you might find nice ideas!
"...one of the most highly regarded and expertly designed C++ library projects in the world." — Herb Sutter and Andrei Alexandrescu, C++ Coding Standards
"Item 55: Familiarize yourself with Boost." — Scott Meyers, Effective C++, 3rd Ed.
"The obvious solution for most programmers is to use a library that provides an elegant and efficient platform independent to needed services. Examples are BOOST..." — Bjarne Stroustrup, Abstraction, libraries, and efficiency in C++
V8 javascript engine is said to be very well engineered. http://code.google.com/p/v8/
Google Chrome also has an interesting architecture and high quality code base. http://code.google.com/chromium/
Both are open source, under BSD-like license.