I wrote recently about leaving Greenpeace. Here's the reply I received from Greenpeace:

I did have a look at your reasons for leaving Greenpeace and found them
rather obtuse.

You say that "One of the best things about them was that they provided
information. Now with the internet I can do all that myself". This is
true to an extent, but a lot of the campaigning work that we do is based
on original research that we have undertaken - for example our research
and exposes on the palm oil trade in Southeast Asia
http://www.greenpeace.org.uk/media/reports/caught-red-handed-nestl%C3%A9
-sinar-mas-and-palm-oil and cattle ranching in the Amazon
http://www.greenpeace.org.uk/blog/forests/slaughtering-amazon-20090529
or our work on decentralised energy
http://www.greenpeace.org.uk/media/reports/powering-london-into-the-21st
-century - information and campaigning that you wouldn't find on the
internet unless we'd put it there!

Your say "Also, I don't think they like the same political ideas as me".
I followed your link and found the manifesto link that covered items
such as banning guns and legalizing prostitution, both of which are
outside of our environmental remit. The only items on the list that
were vaguely relevant to us were free market energy and free trade? We
do work with privatised energy companies, for example the green energy
scheme Juice that we set up with Npower, though we might argue that it's
harder to enforce energy efficiency measures or investment in renewables
with disparate private contractors focused on short term profit, rather
than a centralized nationalised energy provider. There is loads of
information on the pitfalls and challenges of free trade at
http://www.greenpeace.org/international/campaigns/trade-and-the-environment

Lastly you say that "I'm concerned that Greenpeace is more like a
proprietary software company but I'd like it to be more like an
OpenSource project" Unfortunately I have no idea what you mean by this.
We very openly share information and campaign skills via our web sites,
local groups and campaign co-operations with numerous other NGOs and
local communities.

That's a good point about research, if Greenpeace don't do the research it won't be there to be read on the net. I'm not sure my reasons for leaving are entirely cogent, but I feel better for it!