Yes, but the Pigmen and Ghasts won't spawn.
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Going to the Nether you can get unique items only found in the nether and monsters. Items only found in Nether are: Netherrack, Soul Sand and Glowstone. The Mobs only in the Nether as of Minecraft V1.8 Beta are: Ghast and Zombie Pigman.
You cant go to Olympus but you can go to the nether.
In order to to the nether you need a portal.
Yes. There are minecraft servers that have skyblock with nether enabled that are cracked.
A Nether portal is an obsidian structure that can transport players to or from the Nether dimension. USB doesn't come into it anywhere. The 'Minecraft USB nether portal' is just a YouTube fake.