I agree 100% with these 2 bars and have been visiting them for many years!
In De Wildeman has arguably the best selection of beers in Amsterdam, usually 18 different draft beers (no Heinekin or Amstel rubbish here!) and about 250 different bottled beers! Not sure which draft beer to try? No problem, talk to the bar staff and they will be delighted to guide you and will also offer you a small taster sample.
Wynand Fockink is something special, located right beside the Old Jansens distillery at the back of the Grand Hotel Kraznapolski, it is a typically small Dutch bar and I guess that most people will walk right past it, if they ever see it because of the lane it is in!!!
The flavoured licqours are poured into small sherry type glasses until only surface tension is holding the licquor in the glass. The only way to drink it is to move everyone out of the way with your bum and bend down over it and slurp some out till you can lift the glass! Don't worry this is accepted practice and makes for interesting conversation with the mostly locals in the bar!
A word of warning though, Wynand is only open between 3pm and 9pm, although if the bar man whose name escapes me is having a good time, he "might" have a lock in!!!
There is also Cafe Belgique
http://www.xs4all.nl/~phj/ another excellent small beer bar.