Olympia, as a thing, is a mod/api designed to add large numbers of blocks, items, and more in a consistent manner. With Olympia you'll be able to register several "flavours" of metal, wood, stone, gem, and so on.
Here's an example: All I've done is provide all the textures that Olympia will need, and registered the metal "Mythril". Here's what an NEI screenshot of all the blocks and items generated by Olympia, just by me typing out one line.
A note before continuing, Olympia generates everything in combinations of Flavours, Patterns, and for blocks: Shapes.
For each flavour registered, it creates blocks for that flavour in each pattern, and for each pattern for each flavour it creates several shaped blocks. First, let's take a look at our shapes.
As you can see, we have several shapes for each pattern/flavour combination. I won't go into detail about what each of them can do just yet, but suffice it to say that your creative palette is VASTLY expanded with these shapes. Stay tuned for more information on these here shapes.
Meet the Content...
Below is just a handful of examples of all the blocks that I've added to my minecraft using Olympia. On default windowed screen size and normal GUI sizing, NEI has 265 pages of Olympia content THUS FAR.
First, the various patterns generated when you register a flavour. This example is Titanium
Brick, Bullion, Machined, Panel, Plate, Tile
Here's what flavours I have registered so far. The example here is JUST the Plate pattern.
Like Metal, stone has it's own Pattern set, seen here. I think for this example I used Granite
Brick, Cobblestone, Flagstone, Hewn, Masonry, Paving, Rough, Smooth, Tile
And the flavours so far, using the Hewn Pattern for example purposes
Wood only uses three patterns (but a fourth pattern is in the works)
Boards, Planks, Slats
And the flavours I have registered so far...
Gem patterns are as follows
Cut, Fancy, Rough, and Smooth
And the flavours registered thus far....
Coming Soon...
Decoration items - items that you attach to the side of blocks for even more decorative power.
Lanterns - a wide palette of styles and colors above and beyond the 16-colors of Vanilla Minecraft.
Large Doors - 3-blocks wide and 5 blocks tall. Huge number of combinations based on the wood and metal flavours.
Special Blocks - wierd stuff that you can only see to believe, including Windmills, Waterwheels, and much more.
Sneak peak...
remember some of those wierd blocks I was telling you about, well, here's just a couple to show off.
On the left is a pair of Avernian Gates (inspired by an old JRPG called Xenogears and it's spinoff Xenosaga). Each registered flavour gets an Avernian Gate. On the right is a few Pylons (obviously inspired by Starcraft and Botania). Each Metal/Gem flavour combination gets a pylon. To the extreme right: a very happy horse.