What I'm using: org-mode + MobileOrg[1] + Dropbox, and a few times a day checking my .org files into a Bitbucket private repo.

And it's all free! Though obviously you could use paid Dropbox and Bitbucket/GitHub accounts as well.

Until fairly recently I used OmniFocus and Evernote, in conjunction with their mobile offerings, but I don't think I'll be looking back anytime soon.

[1] http://orgmode.org/

[&] http://mobileorg.ncogni.to/