Poet is a capable plain text editor with some powerful features.
A tabbed interface allows you to open multiple documents at once, of course. But you're then able to open new vertical or horizontal tab groups, perhaps allowing you to compare documents side-by-side. And dragging and dropping a tab outside the program will see it open as a new window.
A built-in spellcheck highlights errors as you type, while you're able to run web searches (or call up Google Translate) on any selected text in a couple of clicks.
The "Find" and "Replace" dialogs both support regular expressions. There's optional syntax highlighting for many languages (XML, HTML, CSS, SQL, Java, C#, C++, PHP, Ruby, Python, more). You get strong drag and drop support, for both text and files, and Poet supports more than 100 encodings.
Just in case this isn't enough, Poet also provides a few plugins to extend the program even further. Currently you get a rhymer (enter a word and get a list of suggested rhymes) and a regular expression designer, while the Inspector provides basic statistics on word and character frequency.
Poet's core editing and formatting abilities aren't nearly as strong as in most of the competition. There's no code folding, for example. No option to trim blank spaces, or lines. There's no special clipboard management. You don't even get an "indent" option (although you can do it yourself by entering tabs).
If you can get by with a relatively simple editor, though, Poet should prove to be a likeable tool with some useful capabilities, and it's well worth a look.