browser detection & redirection to mobile site

Detect if visitor uses mobile browser and redirect to mobile site (not necessarily via auto-redirect).
Note: it should always be possible for visitor to visit the 'full' site on mobile browser.
Attached two examples:
ANWB: prompts visitor when detecting a mobile browser
BOL.com: auto-redirect, but offers a link to 'desktopweergave' on every mobile page