Eraser Tool

An eraser tool would be a wonderful addition.... It's kind of annoying to have to erase an entire diagram because of one little, erasable error.

This has actually been brought up many times in the past. From what I've gathered, it's far more difficult to implement than you would think. Roll20 uses vectors for the drawings and "erasing" them isn't a simple task due to the way that they function. Even vector drawing programs like Adobe Illustrator suffer from issues with this. You can use the selection tool to grab individual parts of drawings (depending on whether you drew them as a continuous line or as separate pieces) and delete them with the delete key or hit CTRL+Z to undo the last one.

One thing I sometimes do to simulate having an eraser tool is to just use the Freehand brush tool in the same color as my background. Not something that works over maps with complex coloring, but works well enough for blank backgrounds or simple maps.