So, organisations with actual accreditation (as opposed to unchecked web petitions) who have made statements on the matter:
The Intergovernmental Panel on Climate Change:
www.ipcc.ch/
Established by the UN (oh noes) to evaluate the risks of climate change on humans, primarily using peer-reviewed and published scientific literature. They've published three primary assessment reports (in 1990, 1995, and 2001); the has yet to be fully released, but a summary for policy makers has been. It unequivocally states that humans have caused global warming, that is has already had a real effect, and that it has in fact happened more quickly than expected. These reports can only present conclusions that were decided upon unanimously by every scientist on the panel, which includes government-appointed scientists. There is one scientist (Dr. Chris Landsea) who quit the panel in 2005, claiming that it had become politicised and was using unsound science; however, bear in mind he was involved with (and must have endorsed) the findings of the 2001 report (which also confirmed the existence of man-made global warming); his beef is with specifics of the latest report, and not the general conclusions. Several other commentators (including former US Department of Energy member Joseph Romm) have criticised the panel's findings for being too conservative (ie, downplaying the dangers of global warming). I will stress again, for the record, that only conclusions with unanimous consent make it into the panel's reports.
Joint Science Academies:
www.royalsoc.ac.uk/docume...=1&id=3222
The National Science Academies of every G8 nation, plus China, Brazil and India (who are, of course, three of the largest greenhouse gas emitters), issued a statement in 2005 explicitly supporting the IPCC conclusions and opining that the evidence of climate change was sufficiently clear to justify action by nations.
American Meteorological Society:
www.ametsoc.org/policy/cl..._2003.html
In 2003, the AMS released a statement on the matter. Their conclusion can be seen at the link; in brief, while they opined that temperatures have been rising and that human activity is certainly involved, though they stressed that they could not draw exact conclusions on the magnitude of human activity versus natural variance. The first lines of the introductory section are:
"Human activities have become a major source of environmental change. Of great urgency are the climate consequences of the increasing atmospheric abundance of greenhouse gases and other trace constituents resulting primarily from energy use, agriculture, and land clearing. These radiatively active gases and trace constituents interact strongly with the Earth's energy balance, resulting in the prospect of significant global warming."
Federal Climate Change Science Program:
www.climatescience.gov/Li...xecsum.pdf
This was a program commissioned by the Bush administration (certainly no enthusiastic supporter of the scientific consensus on global warming) in 2002 to study the matter. In 2006, they released the first of 21 assessments, which concluded, amongst other things, that there is "clear evidence of human influences on the climate system (due to changes in greenhouse gases, aerosols, and stratospheric ozone)".
The Geological Society of America:
www.geosociety.org/aboutu...tion10.htm
In October 2006, they adopted a statement on global climate change. The first lines of the position statement are:
"The Geological Society of America (GSA) supports the scientific conclusions that Earths climate is changing; the climate changes are due in part to human activities; and the probable consequences of the climate changes will be significant and blind to geopolitical boundaries."
The report also makes note of and endorses similar statements by the American Geophysical Union and American Chemical Society, as well as reference the Joint National Academies of Science report I linked to earlier in the post.
In 2004, Naomi Oreskes made a study of 928 abstracts, published in refereed scientific journals between 1993 and 2003, and listed in the ISI database with the keywords "climate change":
www.sciencemag.org/cgi/co.../5702/1686
75% of said abstracts either explicitly or implicitly were in support of the consensus view; 25% dealt with matters that didn't directly provide an opinion either way. Not a single one argued against the prevailing scientific view that human activity is causing climate change.
I will repeat: not a single one. This study was rather broadly publicised.
There is exactly one major scientific organisation that denies that human activity is the primary cause of observed climate changes:
dpa.aapg.org/gac/papers/c...change.cfm
That being the American Association of Petroleum Geologists.