It's about time to announce something I've been working on for about three months now: libgdata. It's a GLib-, libsoup- and libxml2-based library for accessing GData APIs, as used by most Google services. There already exist several such libraries in a variety of languages, but as far as I'm aware this is the first one written in C — and thus the first which is widely accessible to the GNOME stack. So far it has decent support for YouTube video queries, and the beginnings of Google Calendar support.
Having ported the Totem YouTube plugin to use libgdata, my next plan is to port the evolution-data-server Google Calendar backend as well. With that done, libgdata will hopefully be stable and fully-featured enough for people to get to work on starting to fulfil Rob Bradford's dream of tighter desktop integration with web services.