If you have gmail, you can add Google Labs Tasks. Nesting, one-click add from email, etc.

And integration with Google Calendar, which is a godsend.

I've been using VitaList, and the best part is that they have an API (JSON or XML). I made a small app that fills in the one thing Vitalist doesn't have, auto scheduling. I know there's calendars, but I like for some things to just show up in my todo at certain times of a month, or once every few months, or every other week and putting 'Do Laundry' on every friday in my calendar is no good. Calendars don't auto roll tasks onward too, and I might not do my laundry on Friday, but by fridays its time to do that in the upcoming few days, so a rolling todo is better. VitaList is iPhone friendly as well, so I get my todo to go.

Once I thought they had lost my data. Turns out if you change your password and log back in with your old password, you get some weird old data from I don't know when -- this resulted in my recreating about 75% of VitaList on a Friday, and then realized I had not lost my data and stopped. I'm keeping that code around, just in case they start charging for the API or something... :).

edit: one more thing, i find the view by context feature really handy - my contexts are "home" "work" "errands" - and being able to quickly group by that (or by project) is nice.

I've been using Tracks (http://getontracks.org/) now for quite a while and am pretty happy with it. The downside is that you have to host it yourself or pay a small fee for a hosted version. But it's pretty powerful, implements the general GTD stuff without being slavish, and has some nice new features like recurring tasks and a iCal link. Not sure what your needs are for nesting exactly - most to-do apps allow categories/lists/projects, but most don't let you do true hierarchies.

I used Tracks for to quite a while. I liked that you can host it on your own machine. I have a problem with some hosted site for my todo's. I too am looking for something that does hierarchies.

Nothing beats Rememberthemilk. Its UI is very well designed and has integration for many services/devices.

My only complaint is pro version doesn't offer much except syncing.

Doris lets you organize your tasks into groups, and it has some cool features like email integration: http://beta.dorisapp.com/en/

For todo lists, I've been very happy with Toodledo (http://www.toodledo.com/). The free version suits me just fine, and I use it in combination with the free webapp for the iPhone. Unfortunately, the iPhone native app isn't free... Good luck!

Agreed. Using Dropbox and batch files to automate backups saves me a bunch of time.

Try Tonido Workspace. It has horizontal information schema and nesting feature u are looking for.