I'm using the Philips Universal USB AC Charger. It has 1000mAh. It can only charge the N900 around 50% and you have to be careful to plug it out after it reached the above charge otherwise it discharges the N900.
But I like it because it's compact and has the power adapter in the device as well so I can use it as power adapter too (thus when you charge your phone the battery in the charger charges too, and then you just have to take it w/ you as well)