Remember the Milk (https://www.rememberthemilk.com/ https://www.rememberthemilk.com/) is a fairly advanced todo application which works identically offline and

Remember the Milk (https://www.rememberthemilk.com/ https://www.rememberthemilk.com/) is a fairly advanced todo application which works identically offline and online. I believe they use Google Firebase Cloud to automatically synchronise data between all devices and platforms.