A couple of demos came out on Steam earlier this month we failed to note at the time. There's a demo for Flashback and another for Alien Rage (thanks Frans for the reminder). There are no particulars on the demos, so here's word on each game:

Flashback
20 years after the original game’s launch, Conrad is enlisted back to active service for the triumphant return of one of the most acclaimed franchises in gaming history. It’s not a reboot. It’s Flashback re-imagined. The original Core-Team revamped their own creation, leveraging the best of today’s technology while remaining true to the classic side-scroller that set the standards of the genre in the 90’s.

Alien Rage
Unleash a fury of powerful weapons as you blast your way through armies of Alien forces hell-bent on taking you down! Battle through 14 intense levels and prepare to face a barrage of beastly-sized bosses that will test your every skill. Each of your 10 weapons comes with two firing modes and you’ll need all of them! Battle hardened Alien forces will attack you from every direction using various flanking and attack moves. Built with powerful up-to-date graphics technology, awesome physics, destructible environments… experience a truly intense, oldschool styled shooter game!