Wikipedia:Keyboard_layout has images of keyboard layouts from different countries.
http://www.microsoft.com/globaldev/reference/keyboards.mspx can display different keayboard layouts.
Also see OLPC keyboards.
US#
All special markup is easily available.
UK#
All proposed wikicreole markup characters are visible on UK keyboards; accessible either by single key press or Shift+ key press.
PL#
The backslash (\), pipe (|), square brackets (), curly braces ({}) and single quotation mark (') require pressing an additional function key on polish(214) keyboard. The more popular polish (programmers) keyboard layout is practically identical to the US layout.
DE#
Nearly all markup needs an additional key, either Shift+key or AltGr+key. But at least all the symbols are visible on the keyboard.
FR-fr (AZERTY), FR-ch (QWERTZ)#
All ASCII characters (32-127) are available. Some markups need an additional key (AltGr+key, or Alt+key on Mac). A few of them (the potential ^ and ~) must be pressed twice since they are considered as modifiers for other letters.
FR-ca, EN-ca (Canadian multilingual)#
The Canadian French keyboard comes in several flavors. Nevertheless, all proposed markups are available. Some markups need an additional key (Shift+key or AltGr+key). Some of them (depending on the chosen layout) must be pressed twice since they are considered as modifiers for other letters. There is a problem however from French Canadians purchasing US-English keyboards (most of them are outside of the province of Quebec). If they change their layout for a French Canadian one, important markups (like square brackets ([]), greater/lesser-than (<>), slash (/), curly braces ({})) are not at their respective places anymore. These people must switch back and forth between two layouts.
IT#
Square brackets ([]), at-sign (@), number-sign (#) are given with AltGr.
Curly brackets ({}) are given with both AltGr and Shift pressed (same keys as square brackets). Often they're not shown on their keys. Most users don't know how to get them.
Tilde (~) is not available. You have to go through <Press-Alt> + Num1 + Num2 + Num6 + <Release-Alt>. On laptops you need to switch to NumLock, first.
Backtick (`) is not available. You have to go through <Press-Alt> + Num9 + Num6 + <Release-Alt>. On laptops you need to switch to NumLock, first.
See http://en.wikipedia.org/wiki/Keyboard_layout#Italian.
RU#
The only punctuation characters available on the russian keyboard are:
!"№;%:?*()_+-\/.,
All other characters are only available after switching to the us layout.
sv_SE#
^~`are extra hard since they require two keystrokes (plus shift or AltGr) as they are placed as modifyer keys.
\|require combination with AltGr key. The rest require Shift key.