Starting points: EBT links page * Bill tracking on Wikipedia (Eng.)
I've sorted the sites by continent and alphabetically (when applicable, I've provided links to English-language threads concerning specific sites):
Asia
Message in a Bill -

Europe
Grn Trip -

Otkudadengi -

North America
¿On' Ta' Benito? -

Where's George -

Where's Willy -

Oceania
Money Tracker -

Other options (I'd recommend using the sites listed previously as they have larger user-bases, but I feel I should also mention these):
Canadian Money Tracker -

Track Dollar -

Inactive or defunct:
www.currencypath.com - An ambitious tracking project which had a number of "ups and downs" before going offline indefinitely.
www.wheresmymoola.co.za - South African rand notes could be tracked here. Currently down.
www.osatsu.net - A tracking site for Japanese yen bills once lived here. The URL has been acquired for other purposes.
www.trackgandhi.com - This domain was registered with the intention of tracking Indian rupee notes, but it had a very short existence.
www.wheresnt.com * www.wheresrenminbi.com - These two "Wheres" domains have expired, leaving us with fewer tracking opportunities in Asia (Taiwanese dollar and Chinese yuan).
www.cashfollow.ch - A site for tracking Swiss franc notes which had a long existence. The URL has been acquired for a new purpose.
www.cupura.ru - The first Russian ruble bill tracking site.
www.doshtracker.co.uk - Another site (pound notes) with a checkered history, currently down.
www.findlizzy.co.uk - This site had a decently-sized userbase of British pound trackers, but that, sadly, did not stop its demise.
www.kovesdapenzed.hr - Hungarian forint.
www.moja-lova.com - The first Croatian kuna notes tracker.
www.roobex.ru
www.sek-tracker.se - The original Swedish krona bills tracker. The site is still up, but only seems to contain information about the project.
www.sedlarna.se - Another old tracking site for kronas.