Arena is a powerful tool for playing chess and analyzing games.
The program interface is better than many similar tools. The 2D board looks good and is very configurable, key options are relatively easy to find, and you can learn a lot about Arena's features just by browsing the menus. There's detailed local help, too (and a real Windows Help file, not a PDF or a website with no search feature).
Arena is able to use more than 250 chess engines, many of which have adjustable strength, so you're unlikely to run out of opponents. Although if you do, it's also possible to play online games against people worldwide.
The program has strong support for analysing games, helping you understand the thinking processes of individual engines.
A host of extras includes support for variant games, including Shuffle Chess and Fischer Chess/ Chess 960. There's a stack of configuration options to ensure everything works as you'd expect. And if you find yourself getting too engrossed when you should really be doing other things, don't worry - a built-in Boss Key (Ctrl+0) can dismiss Arena to the system tray whenever someone gets too close.