Can you use sock yarn for other projects?
It is perfectly usable to make nice hats, mitts, shawls, lightweight garments, baby blankets, blankets (see all the sock yarn blankets being made!) and whatever else you care to work it into.
What can I knit with one skein of sock yarn?
Sheep Among Wolves has collected a great bunch of patterns for all sorts of things you can make with a single skein of yarn, none of which are socks. There are scarves, shawls, cowls, boot toppers, baby clothes and more, all of which you can make with just a bit of yarn.