The 'small' (6-pin mini-DIN, A4000D) and 'big' (5-pin DIN, A2000, A3000 & A4000T) Amiga Keyboard connectors have the exact same pinouts as equivalent PC-keyboard connectors, so you can use 'standard' PC keyboard adapters. These can be found in both 'directions' ('small' connector on the keyboard -> 'big' connector on the computer or vice versa).
http://www.komplett.no/k/ki.asp?sku=100158&cks=PRL'big'->'small'
http://www.komplett.no/k/ki.asp?sku=100159&cks=PRL'small'->'big'
A PC keyboard-switcher should also work with Amiga keyboards. But do go for a slightly more expensive one (ie electronic), because the cheap mechanical ones are equivalent to hot-plugging the keyboard which is not a nice thing to do to your Amiga.
Mouse switching is also possible. But this will be expensive, because you'll need one of these
http://amiga.resource.cx/exp/search.pl?dir=misc#topolino3 (or similar) for each of your Amigas, and ofcourse a PS/2-style PC-mouse.
-Paul
