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This is mainly just a FYI type post, but earlier this week I discovered Nunjucks, a client-side templating language by Mozilla. I've been pretty much sold on Handlebars as my template language, but Nunjucks has a lot going for it too. Out of the box it seems to support a lot more than Handlebars (inheritance and asynchronous support for example) and the template synax is as friendly as Handlebars'.
Of course, my only real requirement for template syntax is to not suck as bad as Jade but that's just me.
It supports client-side (obviously) and server-side (for Node) so it's ready to go pretty much anywhere. I'd love to see this supported in Harp in the future.
If you want to take it for a test drive, I built an online Nunjucks tester here: http://www.raymondcamden.com/demos/2014/oct/15/test.html. You can't test everything there of course (inheritance for example), but you can quickly test out some of the syntax and see how it feels.