All Couchbase SDKs are Open Source, and most source is Apache 2.0 licensed (there's a little bit of MIT license in there too). A number of people have stepped forward with new features and fixes, and we hope you will too. This should help you get started.
Issue Tracker, Source Code & Code Review
Maybe you want to jump in and read some code? Or, you know your git, your gerrit and you are itching to file and fix a bug? Here are the coordinates you seek.
If you're new to all of this, there's more of an introduction below.
Java
Issues: http://www.couchbase.com/issues/browse/JCBC
Source: https://github.com/couchbase/couchbase-java-client
Code Review: http://review.couchbase.org/#/q/project:couchbase-java-client,n,z
.NET
Issues: http://www.couchbase.com/issues/browse/NCBC
Source: https://github.com/couchbase/couchbase-net-client
Code Review: http://review.couchbase.org/#/q/project:couchbase-net-client,n,z
PHP
Issues: http://www.couchbase.com/issues/browse/PCBC
Source: https://github.com/couchbase/php-ext-couchbase
Code Review: http://review.couchbase.org/#/q/project:php-ext-couchbase,n,z
Ruby
Issues: http://www.couchbase.com/issues/browse/RCBC
Source: https://github.com/couchbase/couchbase-ruby-client
Code Review: http://review.couchbase.org/#/q/project:couchbase-ruby-client,n,z
C/C++ (libcouchbase)
Issues: http://www.couchbase.com/issues/browse/CCBC
Source: https://github.com/couchbase/libcouchbase
Code Review: http://review.couchbase.org/#/q/project:libcouchbase,n,z
Python
Issues: http://www.couchbase.com/issues/browse/PYCBC
Source: https://github.com/couchbase/couchbase-python-client
Code Review: http://review.couchbase.org/#/q/project:couchbase-python-client,n,z
How to Contribute
As with all Open Source, there are many, many ways to contribute. Some of these don't even involve submitting code. Do you use the client library and you noticed something that could be an issue or noticed a bug? Then you can jump in and file an issue.