The ASF is made up of more than 150 top level projects which cover a wide range of technologies. Chances are if you are looking for a rewarding experience in Open Source, you are going to find it here.
Apache Deltacloud provides common front-end APIs to abstract differences between cloud providers.
Deltacloud contains a cloud abstraction API - whether the Deltacloud classic API, the DMTF CIMI API or even the EC2 API. Each abstraction API works as a wrapper around a large number of clouds, shielding users from their differences. For every cloud provider there is a driver "speaking" that cloud provider's native API, freeing you from dealing with the particulars of each cloud's API.
XPath for Java Objects
A Java-based implementation of XPath 1.0 that, in addition to XML processing, can inspect/modify Java object graphs (the library's explicit purpose) and even mixed Java/XML structures.
Client-side HTTP implementation in Java.
Commons HttpClient is a library for client-side HTTP communication. It provides support for HTTP/1.1 and HTTP/1.0, plus various authentication schemes and cookie policies. Thanks to it's widespread use and years of development, it is a very mature and stable codebase. However, due to limitations in the API design, Commons HttpClient will eventually be replaced by HttpClient 4.0 with a completely redesigned API based on HttpCore.