Hard drugs are illegal in Amsterdam. Not that you can't get them, but just know - they are.
You can legally get weed (of course), hash, and *undried* mushrooms. Naturally, you can get them dried anyhow, but going by the letter of the law, they can only sell you fresh 'shrooms.
Weed-wise, being used to California-style chronic (or rather, not really, because I don't like it), the stuff in Amsterdam didn't seem strong at ALL, like everyone else said, but it was very tasty, clean, and paranoia-free. Look for a reputable coffeeshop (don't go to the tourist traps) and ask for a menu. Politely ask the counterperson what they recommend.. and hopefully you won't look like a tourist dupe and they'll hook you up with some tasty stuff.
Personally, I liked the hash in Europe much better, but that's just a personal preference.