UltraJSON is a fast and extendable JSON encoder and decoder written in pure C

Python bindings are available as the module ujson (through easy_install / pypi):
http://pypi.python.org/pypi/ujson/


Installation instructions:

1. Build and install ujson Python extension (requires root)
Go to <root>/python
Type: python setup.py build install  

2. Run tests (as needed)
Type: python tests.py

Same instructions applies for Windows except that step 1) isn't necessary since
a prebuilt static library is included.

Preliminary benchmarks:

64-bit benchmarks Linux
Python 2.6.6 (r266:84292, Sep 15 2010, 16:22:56)
OS Version: Ubuntu 10.10
System Type: x64-based PC
Processor: Intel(R) Core(TM) i5-2300 CPU @ 2.80GHz
Total Physical Memory: 4096 MB

Array with 256 utf-8 strings:
ujson encode      : 2714.66499 calls/sec
simplejson encode : 1542.63718 calls/sec
cjson encode      : 132.23604 calls/sec

ujson decode      : 2079.17287 calls/sec
cjson decode      : 992.21602 calls/sec
simplejson decode : 278.92061 calls/sec

Medium complex object:
ujson encode      : 17849.80356 calls/sec
simplejson encode : 3524.32372 calls/sec
cjson encode      : 2967.34656 calls/sec

ujson decode      : 11685.87610 calls/sec
cjson decode      : 8206.67906 calls/sec
simplejson decode : 6549.99750 calls/sec

Array with 256 strings:
ujson encode      : 38543.50303 calls/sec
simplejson encode : 19436.45772 calls/sec
cjson encode      : 12392.55614 calls/sec

ujson decode      : 27207.33157 calls/sec
cjson decode      : 30237.60827 calls/sec
simplejson decode : 25271.93073 calls/sec

Array with 256 doubles:
ujson encode      : 6027.45931 calls/sec
simplejson encode : 2915.54871 calls/sec
cjson encode      : 3546.88804 calls/sec

ujson decode      : 28045.13375 calls/sec
cjson decode      : 15066.73209 calls/sec
simplejson decode : 15604.98222 calls/sec

Array with 256 True values:
ujson encode      : 187342.39634 calls/sec
simplejson encode : 48972.93887 calls/sec
cjson encode      : 67274.93082 calls/sec

ujson decode      : 158103.79663 calls/sec
cjson decode      : 83237.88990 calls/sec
simplejson decode : 115645.98241 calls/sec

Array with 256 dict{string, int} pairs:
ujson encode      : 25301.85690 calls/sec
simplejson encode : 5734.29472 calls/sec
cjson encode      : 4447.73411 calls/sec

ujson decode      : 16290.72288 calls/sec
cjson decode      : 12528.56060 calls/sec
simplejson decode : 10394.23358 calls/sec

Dict with 256 arrays with 256 dict{string, int} pairs:
ujson encode      : 87.40865 calls/sec
simplejson encode : 17.07889 calls/sec
cjson encode      : 17.25164 calls/sec

ujson decode      : 45.94026 calls/sec
cjson decode      : 34.60225 calls/sec
simplejson decode : 26.92238 calls/sec

32-bit benchmarks Windows
Python 2.6.6 (r266:84297, Aug 24 2010, 18:46:32) [MSC v.1500 32 bit (Intel)]
OS Version: 6.1.7601 Service Pack 1 Build 7601
System Type: x64-based PC
Processor: Intel(R) Core(TM)2 Quad CPU Q9550 @ 2.83GHz 2.83 GHz
Total Physical Memory: 8191 MB

Array with 256 utf-8 strings:
ujson encode      : 1191.98175 calls/sec
simplejson encode : 1013.98279 calls/sec
cjson encode      : 1040.66063 calls/sec

ujson decode      : 1215.66875 calls/sec
cjson decode      : 493.30484 calls/sec
simplejson decode : 269.85512 calls/sec

Medium complex object:
ujson encode      : 10307.63723 calls/sec
simplejson encode : 2534.94769 calls/sec
cjson encode      : 2047.95118 calls/sec

ujson decode      : 7274.10026 calls/sec
cjson decode      : 3575.39307 calls/sec
simplejson decode : 3565.51252 calls/sec

Array with 256 strings:
ujson encode      : 21348.25210 calls/sec
simplejson encode : 15736.74638 calls/sec
cjson encode      : 6371.26334 calls/sec

ujson decode      : 26050.25316 calls/sec
cjson decode      : 16468.88215 calls/sec
simplejson decode : 21115.75770 calls/sec

Array with 256 doubles:
ujson encode      : 26975.49110 calls/sec
simplejson encode : 2046.29746 calls/sec
cjson encode      : 2133.56594 calls/sec

ujson decode      : 28430.33722 calls/sec
cjson decode      : 4114.36400 calls/sec
simplejson decode : 4419.08507 calls/sec

Array with 256 True values:
ujson encode      : 89846.12897 calls/sec
simplejson encode : 34288.36862 calls/sec
cjson encode      : 47168.35849 calls/sec

ujson decode      : 99423.47549 calls/sec
cjson decode      : 58795.91460 calls/sec
simplejson decode : 76296.14699 calls/sec

Array with 256 dict{string, int} pairs:
ujson encode      : 14776.41614 calls/sec
simplejson encode : 3876.86634 calls/sec
cjson encode      : 3050.65343 calls/sec

ujson decode      : 12934.39432 calls/sec
cjson decode      : 7993.04345 calls/sec
simplejson decode : 7152.09475 calls/sec

Here is the benchmark run from a 32bit CentOS 5.6 (Python 2.4) machine:

Array with 256 utf-8 strings:
ujson encode : 1453.30891 calls/sec
simplejson encode : 658.31181 calls/sec
cjson encode : 62.18416 calls/sec

ujson decode : 1016.58767 calls/sec
cjson decode : 455.28550 calls/sec
simplejson decode : 124.20439 calls/sec

Medium complex object:
ujson encode : 6010.21634 calls/sec
simplejson encode : 1418.77823 calls/sec
cjson encode : 1252.92530 calls/sec

ujson decode : 4637.52630 calls/sec
cjson decode : 3444.13604 calls/sec
simplejson decode : 2166.18641 calls/sec

Array with 256 strings:
ujson encode : 12252.28889 calls/sec
simplejson encode : 9351.67532 calls/sec
cjson encode : 7786.13697 calls/sec

ujson decode : 10951.17394 calls/sec
cjson decode : 15971.02425 calls/sec
simplejson decode : 6796.77480 calls/sec

Array with 256 doubles:
ujson encode : 16300.61218 calls/sec
simplejson encode : 1613.39428 calls/sec
cjson encode : 2035.58937 calls/sec

ujson decode : 17301.00746 calls/sec
cjson decode : 5785.33627 calls/sec
simplejson decode : 6199.49364 calls/sec

Array with 256 True values:
ujson encode : 72618.15350 calls/sec
simplejson encode : 18707.57593 calls/sec
cjson encode : 24150.26201 calls/sec

ujson decode : 53650.94162 calls/sec
cjson decode : 48069.53050 calls/sec
simplejson decode : 47098.40293 calls/sec

Array with 256 dict{string, int} pairs:
ujson encode : 8811.85922 calls/sec
simplejson encode : 2756.91262 calls/sec
cjson encode : 1758.26962 calls/sec

ujson decode : 6490.36358 calls/sec
cjson decode : 6330.77263 calls/sec
simplejson decode : 4161.97048 calls/sec

Dict with 256 arrays with 256 dict{string, int} pairs:
ujson encode : 31.08834 calls/sec
simplejson encode : 10.41434 calls/sec
cjson encode : 6.93790 calls/sec

ujson decode : 19.81373 calls/sec
cjson decode : 20.31727 calls/sec
simplejson decode : 15.05690 calls/sec


See (python/benchmark.py) for further information.

NOTE: These benchmarks are preliminary!