Oh hai: My ears are burning… Thanks for the @ mentions… Indeed, my app grupz.com (among other things) generates widgets that can be embedded on other sites. This is done via iFrames.
In terms of making things embed nicely, you have to construct embed code for your users that does the right thing. Unfortunately, they do sometimes mess it up.
I’ve also posted about trying to add embed.ly support for my widgets, but I’ve not got it working yet. (That would ultimately make embedding our widgets more-or-less doofus-proof! 
But there are some difficulties with building the required API components for that directly in Bubble – search embed.ly in the forums and I’m sure my description of that will come up.)
You can see examples of these widgets in action on the GRUPZ site:
Also, I just added support for a BUNCH of languages to the calendar widgets. See:
I’m posting an article over in the showcase section that describes (a bit) how I did this momentarily. There’s a lot of cool stuff you can do with Bubble and a little JavaScript library elbow grease.
-K-