This is the program which will be launched if you choose “View Source” in Opera at any time. I highly recommend (at no financial gain to myself), Ultraedit (Windows only) or BBEdit (Mac only) or TextWrangler (also Mac only).
Mac users: the default text editor in OS X, cunningly named TextEdit, will try to render HTML, and is therefore a poor choice for viewing source, where you presumably want to see the markup itself. I highly recommend that you install TextWrangler or BBEdit.