Apple - Mac OS X
I prefer OS X to XP, but, darn it, I wish they could get their network straight. In the 14 or so point releases I've been through, Apple had two where networking worked.
On occasion I get an OS X point release that handles plugging in an ethernet cable correctly. OS X switches from using my 802.11b connection, correctly manages the switchover to 100mbps ethernet, and moves along. Lately 10.3.8 manges the switch, but runs at 802.11b speeds!