Click to expand.I would agree that TextEdit with Rich Text Format disabled is a good analog to the Windows Notepad. At the same time, TextWrangler that myflynn recommended is a good tool.
It much more powerful and more like the freeware NotePad that is available for Windows, but it might not be simple enough for some applications. I have NotePad, WordPad, and NotePad withing easy reach on my Windows machines and both TextEdit and TextWrangler on my Mac's Dock. I find I use TextEdit much more than TextWrangler.
Like Notepad++, there’s a thriving community who have built a variety of plugins for increased functionality. Some of the best are the JavaScript packer, the SQL code reformatting script, and the text-to-table adaptor.
It's a case of which took best fits the job at hand. Extra features can sometimes get in the way.