Socks are the perfect portable project & make wonderful, useful gifts. Although there are a few key techniques to learn, once you get going, they become somewhat mindless & can be the ideal compliment to a more complicated project.
Additionally, I love socks for these reasons:
I get more wear over time out of these projects than any other garment.
They make great in-law gifts.... heck, everyone wears socks!
They are portable and perfect for long car rides or airplane trips.
I can try new techniques (cables, entrelac) without committing to a huge project.
They don't have to be perfect - they are on my feet!
In general, you'll want to use machine washable yarn, though I've learned that handwash yarn can be conveniently washed while taking a shower :). Heavier yarns (such as Lornas Laces Shepherd Worsted) make great socks for colder climates - and for wearing in boots, while the fingering weight yarns can be used to create custom dress socks. Either way, you're sure to be pleased.
As far as supplies are concerned, Socks were traditionally knit using 4 or 5 double pointed needles. In the last few years, however, 2 new techniques have been popularized that use longer circular needles. The Magic Loop uses 1 40" needle, whereas the other technique (Socks Soar on 2 Circular Needles) calls for 2 24" circular needles of the same size.