Text editor capable of viewing invisibles?

In Visual Studio's Open File dialog, the Open pushbutton has a down arrow next to it that lets you choose Open With.... One of the options in the resulting dialog is Binary Editor.

I've used this now and then to spot some invisible character or to resolve some line-ending issue.

vim (in either textual or graphic mode) can show all control characters if you :set list. The BOM is a special case, controlled by the :set bomb or :set nobomb commands.