On Firefox and Safari ⌘R refreshes the page
F5 also seems to work for Firefox, but not Safari
And although I do recommend looking through the list linked by Eric, the specific keyboard shortcut you're asking about is Option-Command-Escape, which opens the Force Quit dialog (it's also available under the Apple menu: *→*Force Quit…). If you're looking for process information similar to that given by Task Manager in Windows, open Activity Monitor with Spotlight or from your Utilities folder.