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Since the euro will replace Dutch currency (as well as German, French, Belgian, etcetera) on January 1, 2002, it seems a good idea to node this for the ages now.

The Netherlands have regular coins in 5c, 10c, 25c, 1 guilder, 2 guilders 50 and 5 guilders. I say 'regular' because there have been special editions of higher values, like het zilveren tientje (a silver 10 guilders coin).
The cent is the base value, although in the 1980's the 1c coin was taken out of circulation because of inflation. Cash transactions are rounded to the nearest five cents. The 5c coin itself, also called stuiver in popular speech, is made of bronze and is the largest of the cent coins. The 10c coin is a very tiny one and made of nickel, giving it a fake silvery outlook. Ten cents is also called dubbeltje in Dutch, while the 25c coin was named kwartje. This is a nickel coin too, sized between the 5c and the 10c coins.
The 1 and especially the 2,50 guilder coins are rather large and also made of nickel. Guilder is called gulden in Dutch, words which both have the same roots in gold. The 2,50 guilder coin is nicknamed rijksdaalder (etymologically comparable to rix-dollar as I've been told). The coin worth 5 guilders is made of bronze 'though and somewhat smaller and thicker. It is relatively new, since the Netherlands had a green 5 guilders note instead until the 1980's. Because of its young age, this coin has received no nickname in popular speech. Or it would be vijfje (fiver), which actually refers to the old paper note.
All coins have their value on one side and the queen (Juliana or, since 1980, Beatrix) on the other.

Most Dutch are of the not so humble opinion that their paper money is the most beautiful in the world. For one thing it's very colourful (examples are available at http://www.dnb.nl/bankbiljetten/index.htm). De Nederlandsche Bank (DNB) issues notes of 10, 25, 50, 100, 250 and 1,000 guilders.
The 10 guilders note is coloured blue and is nicknamed tientje. The watermark shows a kingfisher. According to DNB there were 80.9 million in circulation on October 31, 2001, making it the second popular note in Dutch money. Just a little less notes there are of 25 guilders, a red note with another bird watermarked: a robin (or redbreast if you like). This red paper money is called geeltje (geel = yellow) because it used to be a yellow note ages ago. Actually, now the 50 guilders note is the yellow one, showing sunflowers, and a bee in the watermark.
The 100 guilder note is the most frequently used, according to De Nederlandsche Bank. They issued 126.9 million of these. It is a brown silvery note with honeycombs. The watermark shows a remarkable little owl with glowing eyes. Synonym for 100 guilders in Dutch is rooie rug (red back), which also goes back to the time that this note was in fact red, not brown. The 250 guilder note is also a piece of art. The purple money shows a typical Dutch lighthouse. Watermarked is a bunny.
The 1000 guilder note is hardly used by any normal person. It circulates mostly in the underworld. In my entire life I've had these in my own hands for three of four times. The note is green with some abstract flower depictions. The watermark contains another bird which can be seen a lot in the Netherlands, called pewit or lapwing.
Besides a watermark, all notes have a series of other characteristics to prevent it from being counterfeited, such as extremely tiny letters which are barely visible to the naked eye. The notes are all of the same size.

The Dutch guilder is worth exactly 0.45378 euro (or 1 euro = 2.20371 guilders).

(Thanks for the remarks, mcai7et2 and AndieX.)