There is now a great open source alternative to Remember the Milk called Wunderlist. The multi-OS source code for Wunderlist is available on github and you could even host your own Wunderlist server to sync between devices in case you are wary of who sees your grocery list. There are native apps for Android and iOS, so the mobile experience is up to par. The only two features I miss from RTM are three levels of priority (as opposed to simply important and ordinary) and multi-user lists so that I can make TODO lists with friends and coworkers.
If Wunderlist is not to your tastes, then you may want to try the social TODO list maker called Astrid (see astrid.com
). Unique to Astrid is a focus on social TODO lists, including multiple levels of sharing, including read-write amd read-only access.