I already use the Start Menu's Search box (pictured here) for launching applications, and finding documents and e-mails on my computer.
With Start++ I can create shortcuts to perform custom searches and application launches. For example, using the default install of Start++, if I type the following into my Vista Search box:
w moose -- will search Wikipedia for "moose"
playartist U2 -- will launch Windows Media Player and play all the music by U2 in my library
Start++ kindly doesn't change the look of your Start Menu, it just adds the new functionality.
I thought it was cool that when I wanted to configure Start++, I just typed "Start" into the search box to find the configuration app. (At this point, I have no idea what my Vista Start Menu shortcut hierarchy looks like (nor do I care).)
Start++ is a great little find, and you can download it here.