What organizer to use?

Question

Software and Internet Services

I need, brothers, an organizer program. To write down tasks and not forget anything.
Now I use Thunderbird and the local calendar (with synchronization via Google-calendar), but it's not very convenient, because I can't do nested tasks there.
What do you want?

• Synchronization on multiple machines (home and work).
  
• nested tasks.
  
• Support for tags and priorities.
  

Operating system? Mainly Windows. But if the organizer also works under Linux, it will be just gorgeous.
Have a mobile app? No, it's not critical: I'm not on the road so much to have time to forget why I left.
Anyone who can give some advice will be great. But if you just read to the end, then here's a nyasha for you.
^__^