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There is functionality in Xorg that allows type special characters, without having to switch to a specific keyboard layout. To enable,

• Click System, Preferences, Keyboard.
• Under Layout Options, expand on Compose key position.
• Choose Right-Win key is compose, click Close.

Now you can type extended characters using the RightWin key (next to AltGr), according to this keyboard settings file. Specifically, the lines that start with GDK_Multi_key are those that we can use here. The Compose key is actually GDK_Multi_key in the above file.
Some examples,

• RightWin + C + = produces €
• RightWin + = + C produces €
• RightWin + C + O produces ©
• RightWin + O + C produces ©
• RightWin + a + ' produces á
• RightWin + a + " produces ä
• RightWin + a + ` produces à
• RightWin + a + ~ produces ã
• RightWin + a + * produces å
• RightWin + a + ^ produces â
• RightWin + a + > produces â
• RightWin + a + , produces ą
• RightWin + e + - produces ē
• RightWin + S + 1 produces ¹
• RightWin + S + 2 produces ²
• RightWin + S + 3 produces ³

For more tips, see EasyLinux - Ubuntu Dapper.