API for software check results for the R package distribution network https://cran.rstudio.com/web/packages/
Or I am no longer involved in maintaining.
API for https://www.fishbase.de/
API for Botanical Information and Ecology Network data http://bien.nceas.ucsb.edu/bien/
API for resolving DOIs to full text URLs (if available)
API for http://plants.usda.gov/java/
API for https://ropensci.org packages, citations, etc.
API for traits data - very much a work in progress - would love your thoughts at https://github.com/sckott/traitdb/issues/1