To put it simply, Krumo is a replacement for print_r() and var_dump(). By definition Krumo is a debugging tool (now for PHP5 only), which displays structured information about any PHP variable [...] it does the same job, but it presents the information beautified using CSS and DHTML.

It also supports output of other data in a "pretty format" like backtraces, included files and a listing of all constants. You can check out a demo of it in action here.