A literature search would provide you a wealth of information!
Bob McGaughey with USFW-PNW in Seattle, is the developer of FUSION and I am sure would hand over the source code for watershed segmentation.
Randy Wynne is at Virgina Tech and developed an IDL virtual machine program implementing a variable window filtering approach Popescu & Wynne (2004). IDL is a fairly transparent language and would be easy to translate to a different language. If you need to track him down, I believe that Popescu is now at Texas A&M.
Finally, Mike Falkowski at University of Minnesota published on using wavelets for tree crown extraction and has an implementation in Matlab that he may be willing to share.
Please keep in mind, it is all about application. Take stock of what system you are working in before settling on an algorithm. There are a few papers that compare methods and demonstrate that specific algorithms work better in certain types of systems. For instance, VWF does not work as well in dense, multi-storied canopy as in open forests.