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In Eclipse you can hit CTRL+/ to comment and uncomment a block of code you have selected.

Or CTRL+SHIFT+C to comment and uncomment a selection using single line comments.

Or switch to block selection mode, make a multi-row zero-column selection and type //

I love Eclipse.

In TextMate (OS X), a Cmd+/ does the same thing as well! To the author's credit, he does mention his hack has "no tool dependencies".