What is the most useful R trick? [closed]

In order to share some more tips and tricks for R, what is your single-most useful feature or trick? Clever vectorization? Data input/output? Visualization and graphics? Statistical analysis? Special functions? The interactive environment itself?

One item per post, and we will see if we get a winner by means of votes.

[Edit 25-Aug 2008]: So after one week, it seems that the simple str() won the poll. As I like to recommend that one myself, it is an easy answer to accept.