1)Before setting out on a dangerous journey away from home, hit a block from the side. Don't break it; just view what direction the cracks go. There should be one going almost straight up, one to the right, and one going almost straight down. Now point the mouse downwards and do the same. Circle around until the block is in the same orientation as when you hit one from the side. The "up" crack (almost) points north! Follow that row of chunks to go straight north. When you want to return, hit a block under your feet, identify south, and head home. Or I guess you could press F3...

2)To avoid vision problems (trying to see ores in dark places is very hard for your eyes, they get tired and vision degrades) change the brightness setting. By default, Minecraft has "gloomy" brightness setting. Set it up (bright recommended). This will save your vision.

3) You can spawn fancy skeleton & undead horses but you do need to have cheats enabled. The commands are: 
Skeleton Horse - /summon EntityHorse ~ ~1 ~ {Type:4}
Undead Horse - /summon EntityHorse ~ ~1 ~ {Type:3}

4)This is handy when exploring caves ;- The fog that you see varies on if its day or night, fog at night is black whiltst fog in the daytime is more gray.

5) If you bonemeal grass and make it tall grass you can hide inside it without hosile mobs seeing you.

6) A single block of glass can absorb almost all of the damage from TNT.

7) Cats & Ocelots don't take fall damage.

8) Hoppers can rob stuff out of chests.

9) Endstone heas the same texture as cobblestone, but its just a different colour!

10) Notch drops an apple on death.

11) Cauldrons can undye dyed leather armour!

12) A full diamond set of armour weighs 1100 pounds approx.