Many people (including me) use a Smart Mailbox in Mail to read their new email in one place without having to hunt through all the folders the mail may already have been filed to automatically. The Smart Mailbox can have a very simple definition: Match All where condition is Message is unread.
The problem with this is that, after some time, this smart mailbox gathers lots of already-read mails, and the only way to get rid of them is to switch to another mailbox and back again. Well, there is another way: Bring up the "Edit Smart Mailbox..." dialog (from the Mailbox menu) and immediately hit "OK" without changing anything. This makes Mail.app reload the mailbox, and since you don't have to switch to another mailbox, it's faster, too.
It would be even nicer to have a shortcut for this, you think?
Start Script Editor and type in the following AppleScript:
tell application "Mail" to activate
tell application "System Events"
click menu item "Edit Smart Mailbox…" in menu ¬
"Mailbox" in menu bar 1 of process "Mail"
delay 1
keystroke return
end tell
Note that you may have to increase the delay 1 in the script to delay 2, since depending on how long Mail needs to draw the dialog (having many criteria takes a longer time), the Return keystroke may come too quickly and be ignored. Save the code somewhere as an application bundle and use Quicksilver, Butler, or another suitable tool to assign a shortcut to it. Additionally, you may edit the info.plist file within the script's application bundle and add this...
<key>LSBackgroundOnly</key>
<true/>
...to the first <dict> element. This will make the script run in the background without even showing up with an icon in the dock.
[robg adds: Here's another way to solve this one. Quit Mail and then open the Keyboard Shortcuts tab of the Keyboard & Mouse System Preferences panel. Click the plus sign at the bottom to add a new shortcut. Set the Application to Mail, and the Menu Title to Edit Smart Mailbox... (include the three dots), then define a keyboard shortcut (I used Shift-Control-E). Launch Mail, and you can update a Smart Mailbox using the shortcut followed by the Enter key (to clear the dialog). You could also use Butler or a similar tool to make a macro to turn it into a true one-keystroke solution.]

