One of the most indispensable tools I have is coilless pins. I didn't really discover them until I'd been knitting for about 6 years and wish I knew about them sooner.

Seaming - I use them in abundance and they are so helpful for keeping stitches lined up in bite-sized segments so I can't get off track. I just seamed a patchwork blanket and used about 150 pins to keep everything lined up. They're also super super helpful for seaming sleeves. Really, just seaming anything.

Stitch markers - they're super thin so won't cause any gaps. Colors galore for identifying different sections. Large enough for most needle gauges.

Marking everything - mark the right side of your work. Mark increases. Mark repeats. Mark whatever you want.

General pins - I will tear a small piece of paper and write what I need on it and affix it to my knitting, or a label to a ball of yarn, or whatever.

Other things - I put my cable lengtheners on them and pinned them inside my needle case. I have also put them on clothes hangers to quickly identify sizes.

I bought like 2400 multicolored pins for about $20 (from Amazon, search for gourd pins). I also spent an afternoon sorting by color which was very satisfying.