Though DAT files, which house data as the shortened name implies, are usually best left alone, here's how you can open them.

Another alternative is to use Apache’s Open-Office, a free suite of office software. Using OpenOffice’s Calc, a spreadsheet program, it opened an old Lotus .wk1 file easily.

What is the deal with those winmail. dat files? I get them in my email all the time, and nothing seems to be able to open them.

If the DAT file you're trying to open is text, you can convert the file into text in the Microsoft Excel program by first saving it as a CSV file.