TextWrangler is a good alternative for Notepad++ on Mac Os X but it quite doesn't have the plugins that notepad++ enjoys.
2 Scripts which I really use:
Format Xml Document:
http://www.bergspot.com/blog/2012/05/getting-xml-tidy-xmllint-to-work-on-textwrangler-4-0/
Format JSon Document:
http://ukitech.blogspot.in/2012/08/format-json-in-free-textwrangler.html
Cross:post from
http://bschandramohan.blogspot.in/2012/10/textwrangler-and-formating-xml-json.html
