My current single player world. Highlights:
-Multiple auto & semi-auto farms (cobblestone, iron, rainbow sheep, chicken, wheat/potato/carrot, sugarcane, pumkin/melon, etc.)
-Suspension bridge over 250 blocks
-Large Castle (furnished & supplied) & City (over 100x100 w/ 55+ buildings)
-'Burj Minecraftia' 140 block tall skyscraper
-Nether Transport System

Details:

'Farm' Area:
> Keep (~25x25) w/ supplies
> Cobblestone generator
> Rainbow sheep farm
> Semi-auto wheat/carrot/potato farm
> Tree farm
> Auto egg farm
> Auto pumkin/melon farm
> Semi-auto sugarcane farm
> Iron Golem Farm
> Pens with pigs and cows
> Underground pen with mooshrooms
> Snow farm (snow golem)
> Suspension Bridge (over 250 blocks long)
Town Area:
> Large Castle (~50x50) includes:
- Nether portal room
- Large storage room
- Automated smelting system
- Decorated areas (bedrooms, living room, etc.) w/ carpet, furniture, etc.
- Throne room
- Large basement excavation to bedrock (in progress)
> 'Burj Minecraftia' skyscraper
- Modeled after the tallest building in the world (Burj Khalifa)
- 140 blocks tall
- 20 terraces
> Japanese Style Pagado w/ Cherry Tree Garden
> Main town (over 100x100 w/ 55+ buildings & 50+ villagers & 6 Iron Golems) includes:
- Gardens
- City Hall
- Blacksmith
- Church
- Inn
- Farm w/ Barn and Silos
- Mansion
- Apartments
- Houses
- Restaurant
- Shops
- Magic Shop
- Public Bath
- Minecart Station
Other Areas:
> Nether transport system connecting all areas
> Far North area includes mushroom island and jungle
> Houses/Bases near 2 of the 3 strongholds
> Mine system under Farm and Town Areas
> Igloo in NW Tundra
> Swamp House
> Upgraded NPC Village

*****Many Building Designs Credited to ColtCoyote*****
Check out his stuff: http://coltcoyote.deviantart.com/