I was on the Small Things blog, and she mentioned she loves her teuxdeux app. I went to the app store and saw a demo of it
I currently use Google Calendar to add my to do's- I like that DH and I can view our calendar (with both of our activities) to know what is going on, yet we can make it separate by creating our own calendars (gym schedule, work meetings, etc). I also like that my work calendar (we use Lotus Notes) syncs with my google calendar, so I can keep all appointments (both work/Lotus calendar and personal) in 1 place so I don't double book nor have to go to a separate calendar to make sure I'm free.
As for to do's, I enter them one by one in my Google calendar as events, and then I color code them based on what kind of entry it is. For example, if I need to run an errand (but not sure when I can do it that day, I'll input "Target- Sunscreen, comb, etc" and color code it in pink since my "Errands" color is pink. When I find time (let's say lunch), I'll drag it into the timeslot and be done.
For me, I like having all my appts and to do's in 1 place. The only thing I don't like is the user interface of Google Calendar on my iphone. The text is really big (to me), and I have to constantly scroll to see what is going on for the day. I wish I could see everything in 1 screen shot.
I'm always searching for better to do apps: currently, I'm using Toodledoo.com. (I've tried PlumLife and a couple of others, but found they didn't work well for me.)
I like that it syncs across each of my devices when I open it and is web accessible if I don't have Wi-Fi or my own laptop/phone available.
I use Errands (on iPhone). It's pretty user friendly (and free!). You create different categories for your schedule. I have Appointments, Events, bills, gym. I add an event to the appropriate category. You can set them for a 1 time only item or set it to repeat. There is a "focus" tab that shows all the items on my list that need to be done within 3 days. (you can customize how far in advance they move to this tab). Then there is a button on the app that shows how many items are in the focus tab so one glance at the phone can show how many things you have to do. you can also set audible alerts when an item is due. You can set by date or date and time.
As far as I know, you can't link to someone else's phone/app, but i've never actually looked into this feature.