Apply-style commands in R

Here's a quick table of what I think are the most useful apply-style commands in R:

Function	Input	Output	Best for
apply	Rectangular	Rectangular or vector	Applying function to rows or columns
lapply	Anything	List	Non-trivial operations on almost any data type
sapply	Anything	Simplified (if possible) or list	Same as lapply, but with simplified output
plyr::ddply	data.frame	data.frame	Applying function to groupings defined by variables

For alternatives to plyr, see this post on StackOverflow.