No matter how many needles crafters have, more is better. A solid set of interchangeable needles is an amazing gift for someone or for yourself. They usually come in their own case, which makes it nice and tidy and easy to store. This can be quite pricy and if you choose to go this route, pick quality needles. There's no point in investing money in something that will break after a use or two. Here are some companies which will not disappoint: Hiyahiya, Knitters Pride, Addi, Chiaogoo, and Lykke. The websites will provide you with information as to where you can get these needles locally. There are of course small businesses on Etsy and the like. Here are a few I wouldn't mind owning myself.
When you consider the needles think about their durability, feel and care.
Wooden needles are fairly smooth and will need an occasional waxing to keep them in tip-top shape. Smaller sizes can break easily. They are comfortable and warm and make a nice sound.
Metal needles are extremely smooth and durable. They're generally not flexible thus can be a bit hard on the hands. Cheaper aluminum needles bend and will never be the same again. They also make a nice clicking sound.
Plastic needles are cheap, light, warm, and super flexible. They vary on smoothness. They're good for arthritic hands but don't have a nice sound.
Bamboo needles are cheap and warm and not particularly smooth. Much like wooden needles, smaller sizes can break easily. They are a great option nonetheless.
Glass needles are neat but heavy and no matter the thickness are brittle. They should be used as decoration and not as a functional tool.
Carbon needles are strong, smooth, and flexible. They're a great choice for smaller sizes. They also make a lovely clicking sound.
Different materials will also affect the gauge. Here's a great article with comprehensive illustrations to show how knitted projects will turn out with different needles.
Let me know which needles are your favorite and why in the comments below!