In your template (let's call it widgetTemplate), where the widget should be rendered use the following function (try / catch just to be safe):

Template.widgetTemplate.rendered = function () { try { twttr.widgets.load(); } catch (e) {} }

Same can be used for facebook widgets (share like etc) - FB.XFBML.parse()

Make sure you have widgets.js loaded in head - sync or async.

For async loading use this code in 'head':