I consider Qt to be more a framework than a library per se. The fact of being written in C++ is probably irrelevant, since you usually don't link to it from other projects (say ncurses, gtk based), you use it to build a Qt application or extend it (like the KDE project).

With boost you usually pick and choose and include parts of the (templated) libraries directly into your project directory, so you don't have to embrace it in full.

wxWidgets too. I also know a lot of guys who use Crypto++ and Botan (both C++ class libraries).

Don't stop there:

bsdiff, bspatch, bzip2, dtoa, hunspell, ICU, JSCRE, libjpeg, libpng, libxml, libxslt, LZMA SDK, modp_b64, Mozilla interface to Java Plugin APIs, npapi, nspr, nss, Pthreads for win32, sqlite, tlslite, V8 assembler, WebKit, WTL, zlib, pcap, winpcap, openssl