This mod, for Minecraft 1.8 with Forge, will cause specified chunks to be pre-generated by the normal generation routines.  This is a very light-weight mod with a single command.

This is useful for when you want to improve performance (no delay for worldgen while playing), or if you have a slow structure generating add-on, or if you want to use an offline structure generator (such as MCDungeon).

• Runs in single-player mode only (because the server thread hangs during generation)
• Generate circle or rectangle as required
• Server thread HANGS while generation is taking place; see logs for progress messages as it runs
• Quicker than using other script-based offline generators as Minecraft is only started once

To use:

1. Start world in single-player mode
2. Switch to Creative mode
3. Use /worldgen to set area to generate. Any of these will work, giving coordinates or radius in blocks -
   • /worldgen [radius]
   • /worldgen [x] [y] [radius]
   • /worldgen [x1] [y1] [x2] [y2]
4. Use /worldgen go to start generation.
5. Wait.  A 1000 block radius can take 20min to generate on a slow server.

 If you use Bukkit, then you can do this using the WorldBorder plugin.