I don't think you can without a mod, although I believe Xau's allows you to change the translucency of water. Also, I thought I saw a small mod somewhere that made glass translucent. Look around!
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I'm a decent Java programmer learning Python, and I'm skilled with server administration, WorldEdit, redstone circuitry, etc. Feel free to show me mod ideas, I'm thinking about starting to make mods, and I'd be happy to give insight as to how a mod could be implemented in Minecraft.
Unless I'm mistaken glass and leaves only support fully opaque or fully transparent pixels. Only ice and water support simitransparent pixels.