Open Automator and select to create a Service that receives no input in any application. Double-click the Launch Application action and select your browser. Save as e.g. Go To Safari. This will work with any web browser. Launch Application will bring it to the front if it's already running.
If you want to also reload the frontmost tab, you need to use AppleScript in supported browsers. For Safari, replace Launch Application with Run AppleScript, and paste the following script code:
tell application "Safari"
activate
do JavaScript "window.location.reload();" in first document
end tell
This will focus Safari and reload the frontmost document.
To focus and reload Google Chrome, use the following AppleScript instead:
tell application "Google Chrome"
activate
reload active tab of first window
end tell
Firefox and Camino don't seem to have scripting capabilities to do this. But you can always fall back to OS X's accessibility API, e.g.
tell application "Safari" to activate
tell application "System Events" to keystroke "r" using command down
This will simply simulate a Cmd-R key press in the program you switched to in the first line.
If you want to select a particular window of your web browser, you can use something like the following, again using UI scripting:
tell application "Safari" to activate
tell application "System Events"
tell application process "Safari"
click menu item "Super User" of menu of menu bar item "Window" of menu bar 1
end tell
end tell
This will programmatically click the menu item corresponding to a specific window in the standard Window menu in most applications.
To assign a keyboard shortcut, go to System Preferences » Keyboard » Keyboard Shortcuts » Services, select the Go To Safari service you just created, and assign a keyboard shortcut there.
Repeat for all browsers you want to do this in.
