Amsterdam is one of the major HFT hubs. You can find more HFT opportunities in Chicago, New York, London, but after those, Amsterdam comes next (together with Sydney, Singapore).
Optiver, IMC, and Flow Traders are bigger players. There's also Webb Traders and DaVinci Derivatives, and smaller offices of other players, e.g. Jump Trading, Mako, and there's always a rumor that DRW is planning to open an office in Amsterdam, etc. etc.. There are also some non-HFT shops but I'm not very knowledgeable about them.
Speaking for software engineering (not quant or trader positions): They're quite selective but if you have a decent resume, you can get an interview and it's not extremely hard to get in. Although, you need to have strong interest in trading, and good CS fundamentals, and ability to think quickly on your feet.
Decently big because of the Amsterdam Internet Exchange I think. There are 2 pretty big players, Flowtraders and Optiver.
From what I know getting in requires a decently impressive resume and passing some tough tests. I also expect it requires a masters or even PhD in a very mathematical field. That is the context where I saw them at the very least.
I feel like I'd stand a decent chance at getting in if I could bring up the required enthusiasm in the interviews. But I do have a decently high opinion of myself.
