External configuration (server and client) for Spring Cloud
Admin UI for administration of spring boot applications
An application metrics facade for the most popular monitoring tools. Think SLF4J, but for metrics.
A library for handling Problems in Spring Web MVC
Just Announced - "Learn Spring Security OAuth":
Security library for Spring Web MVC: OAuth, CAS, SAML, OpenID Connect, LDAP, JWT...
A Java wrapper to run Spring, Jersey, Spark, and other apps inside AWS Lambda.