The QuickExec box is a thin black box just above the Fiddler status bar that lets you type commands instead of clicking on the menu options. Just type CLS when you wish to clear the Web Sessions, use ?word to look for a specific word among the Session URLs or filter URLs that are of a specific size using the commands < size and > size.The neXpert 1.1 Performance Tool allows you to type additional commands in the QuickExec box than the pre-defined ones. One of those nifty commands is savesaz filename that let's you save Fiddler Sessions in a Session Archive (.SAZ) file at a specified path. Read more about it in my recent article Fiddler Tips & Tricks.
Also see:
neXpert - YSlow clone for Fiddler
Accidental discoveries