Are there any exciting offerings currently being worked on, or recently released?
With no guess about what excites you, and not knowing how little of the www you have already looked through, here's some names you could look up:
Wilksch, Delta Hawk, Zoche all announce diesels, though
AFAIK only the first of them has any offer on the market.
And, more exciting as an engine to me, ISTR some Swiss project for a GA Wankel.
But all of these are aviation specific so a bit off-topic in this thread.
And, sorry GtE, I will not say anything negative about the Rotax 4-strokers, flying behind one myself. But they can't claim to be at the front of technology today. OK, they are far in advance of the US'an offer, but that's not saying much. To be at least more or less up to date, Rotax ought to offer fuel injection.