How can I reformat my SD card to use it normally again?
The accepted, top-voted answer is patently wrong. All it does for Windows and Mac is wiping the tiny FAT partition of the SD card, leaving the Ext3 partition untouched, quite contrary to what the asker requested. Still, with 20 upvotes it's unlikely to be overpassed by the next (correct) answer which, while more complex, achieves the goal.
If the majority votes that Earth is flat... what should we do?