Can I detect if my code is running on cPython or Jython?

The most clear-cut way is:

import platform

platform.python_implementation()

'CPython'

By default, most of the time the underlying interpreter is CPython only which is also arguably the most efficient one :)

As sunqiang pointed out

import platform
platform.system()

works for Jython 2.5, but this doesn't work on Jython 2.2 (the previous Jython release). Also, there has been some discussion about returning more operating system specific details for calls like these in Jython 3.x. Nothing has been decided there, but to be safely backwards and forwards compatible, I would suggest using:

import sys
sys.platform.startswith('java')

Which will return True for Jython and False everywhere else (actually in Jython 2.2 or older it returns 1 for Jython and 0 everywhere else, but this will still work fine in if statements and other checks). This call works in Jython at least as far back as 2.1, and will work for the foreseeable future.

In Python versions 2.6 or above (note Jython 2.6 has not yet been released) another option is:

import platform
platform.python_implementation

Which returns 'CPython' for the C implementation of Python, 'IronPython' for IronPython and will return 'Jython' for Jython. Obviously this one isn't backwards compatible below 2.6, but will be forwards compatible.