One of the nifty features that's supported by the new Reminders app in iOS5 is the ability to share reminders with other people. But it's not the only option if you don't want to use Apple's services for everything.
There's a simple guide to sharing reminders over at TNW. The basic process is to create a reminder and then head over the Calendar app in iCloud and share the Reminder using an contact from your Address List.
I've been using a different service called Producteev for a little while now. Although there's no iPad app yet it does work in a browser and there's an iPhone app. An Android version is at the Alpha stage as well and available through the Android Market according to the Producteev developer's blog.
Producteev allows you to tag tasks and create multiple workspaces -- a handy feature for splitting tasks by client or some other suitable division. When tasks are overdue, an email reminder is sent to me and it syncs with Google Calendar so tasks appear there. You can also email tasks to yourself.
The perfect To-Do app is something I've been searching for. What's everyone else using?