Socks are great on-the-go projects. They're lightweight, don't take up a lot of space, and are easy to carry with you for those times when you can sneak in some knitting time on the go. While technically all you need is your yarn, a set of matching needles, and a good pattern, there are definitely some things you can pack in your favourite knitting bag that will make your life a lot easier.
Stitch Markers
Stitch markers make everything easier, but there are a few different styles to be aware of. Most people have a preference for one style, and like most things in knitting, there's no best answer. Markers can be made out of plastic, metal, resin, or even out of yarn, and can be plain or decorated with beads, charms, and tassels.
A variety of distinct markers can be very handy as well. In my own knitting, I will choose coloured markers to indicate purpose. For example, when knitting a sock, I like to use green markers as row counters (1 marker every 10 rows), red markers are always used to mark the midpoints, blue markers are for decreasing, orange markers show horizontal pattern repeats if I'm doing cables or lace, and a yellow marker placed in the sock is meant as a reminder that I've made some sort of alteration to the pattern as written.
Ring markers form a closed figure or loop, and sit on the needle. They can be any shape, but circles are the most common, and are best used to mark out pattern repeats, stitch counts, start of round, or places to increase or decrease. Once placed, however, they cannot be removed until you knit or slip stitches to where they are.
Opening stitch markers are my own favourite style of marker. They can function as a ring marker, but because they can be easily opened, they are very useful as well as progress markers and row counters as well. As an added bonus, if you spot a dropped stitch that you need to sew in or pick up, this style of marker can be inserted into the loop to keep it from unravelling further.
Darning / Tapestry Needles
I try to always have a tapestry needle at hand when knitting. While sewing in ends is no one's favourite task, there are frequently points in your knitting time when you don't quite have time to start another row or section. I try to use those times to weave in ends, so that when I reach the end of my project, they're mostly done. Plus, if you get to the end of your sock and need to do some on-the-go Kitchener stitching, you're all set.
Measuring Tool
Having a reliable measuring tool will give you consistent results, particularly since there are definitely places where knitting is socially acceptable, but pulling off your shoe to try on a sock is not. Having a measuring tool with you is also handy if you have to check gauge. For those of you who like to travel light or forgot their ruler at home, you can also use a bank card as a measuring tool: the short side is 2", and the long side 3.25".
Crochet Hook / Fix-It Tool
Mistakes happen, and there's very little that's as frustrating as having a project with you that you can't knit. To avoid having to stop working on a project when you're on the go, it's a good idea to keep a small crochet hook or fix-it tool with you in your bag. I really like both the keychain fixit and the stitch fixer for this -- both tuck neatly into a notions pouch and are easy to find when you need them.
Wool Wash
So, this one's probably not going to go in your knitting bag, but you'll definitely want to have some at hand at home for when you're done your socks. While most sock yarns are made to withstand cool machine wash cycles, a high quality gentle wool wash can make a big difference in extending the life-cycle of your hand-knits, including socks.
Sock Blockers
Like the wool wash, the sock blockers will stay out of your knitting bag and live at home. Although it's not necessary to block your socks after every wash, it's nice to be able to freshen them up every few wears with a good blocking. Part of the joy in crafting is being able to look at the finished product and feel proud, and the difference in what a piece looks like off-the-needles vs. after it has been washed and blocked really is considerable. When looking for sock blockers (or asking for them as a gift!), be sure to look for something that will let the air flow through: your socks will dry faster, especially in humid weather. Make sure to select the correct size: too small, and your socks won't be properly blocked, but too big and they'll be too stretched!