Stock ones seem ok for the price and the mic and answer button work which is a bonus.
The pinout on the four pin jack is important most is aimed for if-fone which has a different pinout IIRC.

Im using Shure SE-535. Sounds pretty good on the N900.

I don't use a headset...but I use SoundMAGIC E-10s for 'phones. Best sounding budget ones ever IMHO.

Nokia BH-905 here, awesome bluetooth headset.

Nokia BH-905i. Better than awesome.

i'm currently using some crappy earbuds that came with my tablet,
I have to use an adapter though, as they do not work properly on the N900.
They're loud. Sound quality, for that piece of crap that it is, is surprising, but not anything to praise about. Also, left side is louder than right side. Probably something to do with that mic. My friend has the same earbuds that came with his Galaxy Nexus (and he dislikes them) and I checked them to see if it had the same problem as mine, and yes, they all have the left side louder than the right.

Edit: I'm also using the original nokia "rubber things", as those fit better on my ear

The reason I use them is because they're way less muddy than the N900 original headphones, but it's just temporary.

I plan in getting AKG K450.
I tried them on my phone in a local store and I found them actually pretty good, but the price was 100€
Did some research online and it seems to be a good choice. They're around 50 euros on amazon.co.uk so I can't really resist.

Just got my IronBuds, and using em.

Everything is fine at the moment, previously had the CX 300 version but ruined it out when, with the headphones connected and the phone inside the pocket, left the car in a hurry, close the door and wires getting trapped between me and the car xD

And now having the CX 400-II for a year and a half, they are running pretty well (despite built in volume control being a bit insensitive for me, and bass way too amplified in my opinion)

Koss Porta Pro

Meh, I tried using the Samsung ones for listening too, and they're utter crap. Compared with other buds, including the in-box Nokia one (which worked well, until sadly the cable broke somewhere) and cheap Panasonic buds (which I use daily), the Samsung buds are too muddy.

I would buy these, just to take advantage of the lifetime warranty (they are apparently quite flimsy), and because of the price.
Shipping to and from Koss would be a ***** though.

As for headphones, I only use them on long trips (>2 hours). I don't quite like the idea of toting a portable amp or buying expensive low-impedance cans, so I have a Panasonic RP-HTX7 in white.