Community maps can sometimes have furnaces or other things with items in them.
In Solar Frost, there is a piece of coal in a furnace somewhere I believe.
In SG2, there is a room only accessible by updating a block to make gravel fall. This gravel will land on a torch or something and pop out.
Not sure where the door came from, though...but you used to be able to pick up the wooden planks from a broken boat and craft one.
In Teweran, there is a jukebox in the village area. Right click that and you get the Stal disc I believe.
On any map with mobs, you can probably get a skeleton to kill a creeper to get virtually any disc.
On SG1, I found a creative way of getting redstone dust...if you are really creative, you can use that to make a torch and cheat the casino, but it's hardly worth it since you have to sacrifice a few other really hard to get items just for the dust and go all the way to the end of the map for it...I'm not telling how to get it. ;D
You can kill squids in certain maps to get black dye. Use this on leather armor if you like...
You can also get vines and tall grass with Shears. You can place the vines, too...very useful if you want to easily access a high up area.
The possibilities are endless...
If you are on the right community map or just think for a bit, the reality is, as long as the item needed to craft it, or the item itself, is able to be picked up without the server blocking it from being picked up, you can get a ton of things... If I could still pick up wooden planks from broken boats, I could make fence gates, or doors. Even chests...
