jEdit- This is written in Java and it runs on any OS including Mac OS. It is also a free open source project which is a best alternative available for Mac.
Editra - this supports major platform like Windows, Mac and Linux. It supports over 60 programming language and comes with icons themes and loads of plugins.
TextWrangler - a good alternative Text Editing and Manipulating, Programming, OS Platform Integration and Other Useful Features it has like Notepad++.
BBEdit - HTML and text editor for the Macintosh.