JOACHIMR  Top-notch statistical analysis software for an unbeatable price. You do have to invest some time to learn how to work it, but that's well worth the price of admission. Many statistical tools become available on this platform way before others (much more costly ones). It is supported by a wide, global user and programmer base. Get yourself a book to learn how to use it if you are not the adventurous or "I'd rather do this with a command-line" type.  
(Version 2.8.1)

UMIJIN  I downloaded the package, ran the installer and after starting R, the app remains in 'loading R' status interminably. I have to 'force quit' R to get out of it.

I'm running a 20" G5 iMac with OSX10.4.6

Any idea of what the problem is?  
(Version 2.3)

RONAN  A significant upgrade. The interface is now a lot less baffling. The most immediately-visible changes affect the ease with which you can see what is in your workspace and how easily you can access help files.

R is powerful, but has hitherto been daunting in its minimalist interface. This is still a package for the professional, but is now going to be a lot easier to get to grips with.   
(Version 2.0.0)

SHIN WACHI  One of the most robust and powerful statistical package that just happens to be free, as well. Most of its power comes from command line interface, but the object browsers can't hurt. Is it just me or it tends to run a lot faster when run directly from terminal app?  
(Version 2.0.0)