Estimation takes practice. It also takes labor. It takes so much labor it may be a good idea to estimate the time it will take to make the estimate, especially if you are asked to estimate something big.

And a lot of other useful discussion in this essay

• Well, you can use emacs for starters!
• Shell - bash uses emacs keybindings
• Many GNU apps default to emacs keybindings
• Gnome apps (includes Firefox) used to default to emacs keybindings. These days you need to change your user config to tell all Gnome apps to use emacs keybindings (see this freebsd gnome list message)

Windows

Surprisingly Windows is really easy to convert to emacs keybindings in every application. That's right, you can use emacs keybindings in Microsoft Word!! All you need is XKeymacs. It's very configurable per-application.

MacOS X

Conveniently, MacOS doesn't use the control key much, so MacOS X is able to offer emacs keybindings without breaking the standard keybindings in other applications.
• All Cocoa applications support simple emacs keybindings by default (C-a, C-e, C-k etc.). You can get even more advanced bindings (including multi-key C-x bindings). See here, here and here.
• Firefox on MacOS X is unfortunately designed to be Mac Friendly, which means if you're a Unix/Emacs person you're left out. It doesn't even support the basic emacs bindings that Mac Cocoa applications do. Fortunately Firefox is such a dynamic runtime it's easy to rectify. See This mozillazine article for simple instructions.

And yes, I do use all these operating systems every day (including usually at least two brands of Unix)! I don't have any information on emacs keybindings for either BeOS or AmigaOS ;)