Retweet? Post their papers to discussion forums, such as HN or others? If you read it and understand it, email them or tweet and give feedback? Simply comment somewhere to let them know you applaud this?
This is good advice worth more than us here might expect. A bit of chatter about a paper is worth a whole lot. In a specialized field, getting 2 or 3 comments back after publication is 'a lot'. Doubling that to 5 or 6 interesting comments doubles the feedback and interest.