Minecraft – How to set per-world permissions

In my opinion, your best bet here is to use Essentials' GroupManager. It's simple to set up, gives OPs all available commands and lets normal users play with basic permissions. All you have to do is download the Essentials Suite from here, paste the Jar files into your plugins directory and start the server to let it generate the default files.

Then, open plugins/GroupManager/Worlds/World/Groups.yml with your favourite text editor and tweak it a little. By default, new players will be added to the default group, which has no building permissions. If you change build: false to build: true under the info section, they'll be able to place and destroy blocks, and use a very basic set of commands.

groups:
  Default:
    default: true
    permissions:
    - essentials.help
    - essentials.helpop
    - essentials.list
    - essentials.motd
    - essentials.rules
    - essentials.spawn
    inheritance: []
    info:
      prefix: '&e'
      build: false
      suffix: ''

  ...

If you want to give your users some more freedom, change default: false to default: true for the Builder group. You can also remove some commands you don't want them to have while still conserving the other ones, such as essentials.kit and essentials.kit.tools. You can also change the group of the already existing users editing plugins/GroupManager/Worlds/World/Users.yml.

For more advanced Permission management, I'd recommend Permissions 3.x before DefaultCommands.

Edit: Here's a really useful command reference for GroupManager: forums.fragnet.net/minecraft/588-essentials-command-reference.html

No such plugin exists, so I made a quick and dirty custom one I call ModeLock. You can download it here, which should in theory work with all future versions of Bukkit unless something drastic changes.

It will by default block all gamemode changes for all players. To allow a player change into the creative gamemode, give them the permission node modelock.creative. To allow them to become survival, give them modelock.survival. To give them both, you can use modelock.gamemode.