Slimes only spawn in certain chunks. The slime finder is a tool that, given the world seed and a set of coordinates, tell you which chunks will spawn slimes. So you just build a large, lit room under Y = 20 in these chunks, and eventually slimes will spawn there. It still takes a while, I suggest building a waiting room nearby, and trying to find 2-3 slime chunks near each other. Remember that no mob will spawn closer than 24 blocks from you.
I don't have the link with me right now, but if you google "minecraft slime finder" you should find it.
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Minecraft 1.9 pre4!!!
http://www.reddit.com/r/Minecraft/comme ... changelog/
I'm most excited about the enchantments. I was a bit skeptical about the experience, but this system of using the experience to power up breakable tools is great. The variety of enchantments is also really nice. Check out the "axe of silk touch" (when mining a block, gives you that exact block back -- glowstone, webs, redstone/lapis/diamond ore yay!)