Besides Astrid, probably the best known todo list, there are a TON of those apps out there, based on various services, such as ToodleDo, RememberTheMilk, Google Tasks, and so on. Here's a roundup. Please note that this is merely a list, NOT a full review. it's NOT even a complete list.
ToodleDroid http://www.appbrain.com/app/toodledroid/com.hoghollow.TaskList
Due Today Lite http://www.appbrain.com/app/due-today-lite/com.lakeridge.DueTodayLite
Ultimate Todo List http://www.appbrain.com/app/ultimate-to-do-list/com.customsolutions.android.utl
DGT GTD and Todo http://www.appbrain.com/app/dgt-gtd-todo-(alpha)/com.dg.gtd.android.lite
Shuffle (GTD) http://www.appbrain.com/app/shuffle/org.dodgybits.android.shuffle
Got to do Lite http://www.appbrain.com/app/got-to-do-lite/com.slamjibe.android.todo
TaskOS http://www.appbrain.com/app/taskos-to-do-list-%7C-task-list/com.taskos
Gtasks http://www.appbrain.com/app/gtasks/org.dayup.gtask
ActionMethod http://www.appbrain.com/app/action-method/com.behance.actionmethod
ActionFocus http://www.appbrain.com/app/actionfocus/megaup.ActionFocus (short review below)
Routinely http://www.appbrain.com/app/routinely/com.braavos.apps.routinely (short review below)