What Continuous Integration System is most geared?
Among open-source PHP projects, the two most widely-used CI implementations are easily Jenkins (jenkins-ci.org) and Travis (travis-ci.org). Both have significant communities, and you can find examples of using them with Drupal (eg wiki.jenkins-ci.org/display/JENKINS/Drupal+Development and www.vmdoh.com/blog/how-we-test-drupal-7-modules-travis-ci). 

Between the two:


Jenkins' positioning is more like Drupal's -- it's an open-source product with a lot of add-ons/options/features, but you have to think a lot about deployment and picking options. It's useful for testing and many other things (e.g. deployment, nightly tarballs, cron-jobs).
Travis is a proprietary SaaS offered on a freemium basis -- free for public/FOSS code; paid otherwise. It's generally easier to setup because Travis provides servers and bundles more features in the default. It has a more focused/limited range of use-cases.


For some specific features that differentiate them (at time of writing):


Jenkins provides reports -- e.g. when a test fails, it lists out specific failures on a nice web-page. You can see an example of a full report at https://test.civicrm.org/job/CiviCRM-Core-Matrix/CIVIVER=4.5,label=test-debian6-1/lastCompletedBuild/testReport/ . In Travis, this is a long-standing feature-request: https://github.com/travis-ci/travis-ci/issues/239
Travis makes it relatively easy to test with different combinations of key dependencies (e.g. PHP 5.2+5.3+5.4+5.5 and MySQL 5.1+5.5+5.6). In Jenkins, this is possible but requires much more sysadmin skill.


As a rule of thumb:


If you have a large project (with lots of code, lots of tests, peculiar integrations, some budget), then Jenkins' deeper feature-set and configurability make it the better choice.
If you have a small project published on Github (e.g. an individual Drupal module or PHP library with no budget), then Travis's simplicity and bundling make it the better choice.
If your project is in between, then there is no clear rule of thumb; it's a judgment call.


Aside: You may find buildkit.civicrm.org interesting. The summaries and explanations are written for an audience that works with Drupal/Joomla/WordPress, although a lot of technical details would need tuning for a Drupal-only audience (e.g. replace "civibuild" with "drush"; replace "Build Types" with "makefiles").

(NOTE: I left several links as plain-text because stackexchange limits the #links from new users.)