What apps do you absolutely have to use a Mac to run, and there are no decent Windows PC equivalent?

• This is not a religious war. Please be specific and practical
• It doesn't have to be a direct 1-2-1 comparison, but overall usefulness to the task

I'll start off with a few:

• KeyNote -- the animations are quite cool and not available in PowerPoint

• iTune's photo sync -- on Windows it makes copy of all the photos you want to sync, effectively double the space taken up by your photos. On a Mac it's easier as long as you use iPhoto

• Keychain -- a centralized password manager tied to the OS. The benefit of this is you don't have to set a Master Password (like Firefox) which you need to enter when starting the browser. And it doesn't reveal your password (like Chrome, which makes no effort in hiding the password you have stored in Options)

• Time Machine -- 0-configuration backup in the background. Easy interface for restoring a file, or even just a contact in the address book.

• Text-to-speech -- works in any program, and sounds better than Windows computer voice

• Quick View -- press space bar to preview a file. Windows95 had quick view, but was removed.