xmodmap is a very useful tool to modify Xorg's keyboard key mappings. Sample usage scenarios include:

• fixing strange keyboards not fully supported by the predefined maps
• configuring swapcaps and the like
• adding special keys and dead-keys using the Mode_switch key

Use xev to find out the keycode for any key. The list of keysym names can be found in <X11/keysymdef.h>.

Write the samples below into ~/.Xmodmap (traditionally) and have it applied by xmodmap on WM startup using this in it's init file (or ~/.xinitrc or so):

xmodmap ~/.Xmodmap

(Sometimes this is automatically done by the WM.)

This is for ctrl-r (my favourite, since I seldomly use caps lock, but sometimes want ctrl-l as ctrl key), for ctrl-l just replace Control_R with Control_L:

remove Lock = Caps_Lock
remove Control = Control_R
keysym Control_R = Caps_Lock
keysym Caps_Lock = Control_R
add Lock = Caps_Lock
add Control = Control_R

Beware: this really does a swap, so if run again things will swap back.

! define Alt_R to be Mode_switch
! (and Alt_R when typed with shift pressed):
keycode 113 = Mode_switch Alt_R
! now define fancy stuff for keys with Mode_switch pressed
! syntax is: keycode <num> = <regular> <shifted> <mode_switch> <shifted mode_switch>
keycode 10 = 1 exclam onesuperior
keycode 11 = 2 at twosuperior
keycode 12 = 3 numbersign threesuperior
keycode 13 = 4 dollar foursuperior
keycode 14 = 5 percent fivesuperior
keycode 15 = 6 asciicircum sixsuperior dead_circumflex
keycode 16 = 7 ampersand sevensuperior
keycode 17 = 8 asterisk eightsuperior
keycode 18 = 9 parenleft ninesuperior
keycode 19 = 0 parenright zerosuperior
keycode 24 = q Q degree
keycode 26 = e E EuroSign
keycode 30 = u U udiaeresis Udiaeresis
keycode 31 = i I idiaeresis Idiaeresis
keycode 32 = o O odiaeresis Odiaeresis
keycode 38 = a A adiaeresis Adiaeresis
keycode 39 = s S ssharp
keycode 47 = semicolon colon dead_diaeresis dead_diaeresis
keycode 48 = apostrophe quotedbl dead_acute dead_doubleacute
keycode 49 = grave asciitilde dead_grave dead_tilde dead_caron
keycode 54 = c C cent
keycode 59 = , < dead_ogonek 
keycode 60 = period greater dead_diaeresis