I edit a lot of plain text in the course of my work: source code, HTML, etc. I also regularly use regular expressions to massage plain text and HTML into shape, search/replace, etc. Thus a good plain text editor is essential for what I do.
My plain-text editor of choice is TextWrangler which is a simplified version of BBEdit. They are among my most-used tools, every day, day in and day out. Both have a free 30 day trial. And for real geeks, they even have optional command line tools in addition to the superbly useful app.
I have tended to prefer the free TextWrangler for its simplicity even though I bought a license to BBEdit, wishing to support BareBonesSoftware. However, TextWrangler is being sunsetted in favor of BBEdit which is better in that it has even more power, without losing much simplicity.