From Gentoo Linux Wiki

[edit] Keyboard Layout

 Section "InputDevice"
  Identifier  "Generic Keyboard"
  Driver    "keyboard"
  Option    "CoreKeyboard"
  Option    "XkbRules"  "xorg"
  Option    "XkbModel"  "pc105"
# US, German, Dvorak
  Option    "XkbLayout" "us,de,dvorak"
# Switch layout with <ALT>+<SHIFT>
  Option "XkbOptions" "grp:alt_shift_toggle,grp_led:scroll"
# watch out, the commas are important. Assign nodeadkeys to de
  Option "XkbVariant"  ",nodeadkeys,"
 EndSection

Other XkbOptions (the same as in /usr/share/X11/xkb/rules/xorg.lst).

Which keys you can choose for switching between languages in /etc/X11/xorg.conf file:

1. grp:switch — toggle using right alt key, but only until unpressed
2. grp:lswitch — Left Alt key switches group while pressed
3. grp:lwin_switch — Left Win-key switches group while pressed
4. grp:rwin_switch — Right Win-key switches group while pressed
5. grp:win_switch — Both Win-keys switch group while pressed
6. grp:toggle — Right Alt key changes group
7. grp:lalt_toggle — Left Alt key changes group
8. grp:caps_toggle — Caps Lock key changes group
9. grp:shift_caps_toggle — Shift+CapsLock changes group
10. grp:shift_toggle — Both Shift keys together change group
11. grp:alts_toggle — Both Alt keys together change group
12. grp:ctrls_toggle — Both Ctrl keys together change group
13. grp:ctrl_shift_toggle — Control+Shift changes group
14. grp:ctrl_alt_toggle — Alt+Control changes group
15. grp:alt_shift_toggle — Alt+Shift changes group
16. grp:menu_toggle — toggle using "context menu" key on Windows keyboards
17. grp:lwin_toggle — toggle using left win key on Windows keyboards
18. grp:rwin_toggle — toggle using right win key on Windows keyboards
19. grp:lshift_toggle — Left Shift key changes group
20. grp:rshift_toggle — Right Shift key changes group
21. grp:lctrl_toggle — Left Ctrl key changes group
22. grp:rctrl_toggle — Right Ctrl key changes group

Possible signs using grp_led:

1. grp_led — use keyboard leds to indicate group change
2. grp_led:num — Num_Lock led indicates group change
3. grp_led:caps — Caps_Lock led indicates group change
4. grp_led:scroll — Scroll_Lock led indicates group change

if you have problems with setting up your keyboard, /etc/X11/xkb/README.config will be most helpful for you, so make sure to read that file

[edit] LinEAK

If someone has time - please put something about lineak here.

ebuilds of interest:

• lineakd
• klineakconfig
• lineak-defaultplugin
• lineak-kdeplugins
• lineak-xosdplugin

also check out the LinEAK project pages on sf

[edit] XOrg 6.9 or Greater, evdev, and USB Keyboards

The details for setting up evdev support for your kernel and X.org can be found here.

[edit] Logitech Cordless Desktop Pro

Section "InputDevice"
    Identifier      "Generic Keyboard"
    Option          "CoreKeyboard"
    Option          "XkbLayout"     "dk"

    Driver "evdev"
    Option "evBits"  "+1"
    Option "keyBits" "~1-255 ~352-511"
    Option "Pass"    "3"
EndSection

[edit] xmodmap

Now there's still the problem of howto map the X11-KeyCodes to actual keys, usable by your windowmanger. If you dont't want to install additional software, go the xmodmap way. You still have to map the generated keycodes to various functions/applications in your windowmanger(e.g. by using khotkeys for KDE). If you install xbindkeys you can assign applications to your keys in a non-windowmanger-specific way. You can even map shortcuts and mouse-events to your keys, if you use xte.