I've been using Poco for some time now and I'm extremely pleased with how complete the library is. I have been programming in Qt for some time and I have to say, if your creating any type of non-gui application, Poco is almost certainly the way to go. I've used it to create cross platform stand-alone web services and it works wonders.
I especially enjoy how granular the libraries are. It allows me to create a leaner application then anyone would ever dream of with Qt.