There are several efforts underway to implement XProc, in both the open source community and in commercial products.
If you're an implementor and you'd like your implementation listed on this page, please let me know how you'd like it described and I'll add it.
EMC’s Java-based XProc processor. The processor features an extensible architecture and is easy to embed in other Java applications. Free for developer use.
Tubular is a Java implementation based on immutable objects, in order to facilitate the addition of parallelism support, thus reducing the need for locking mechanisms.
XProcerity is a Java implementation focused primarily on high performance in multi-threaded environments, such as high-traffic enterprise web applications.
Jim Fuller's xprocxq is an experimental bootstrap implementation of W3C XProc Draft Specification, written in XQuery, for the eXist XML Database.
An official test suite for XProc is under development. See http://tests.xproc.org/ for more information.