This is far from a comprehensive or fully vetted list, but here are some ideas off the top of my head on the improving institutional decision-making front:

• The Alliance for Useful Evidence
• The UK What Works Centres/Evidence Quarter, particularly the What Works Centre for Wellbeing
• Society for Judgment and Decision-Making
• Society of Decision Professionals
• Behavioural Insights Team and ideas42 (mentioned below by Mati)
• Campbell Collaboration (research syntheses of social science literature)
• Cochrane (research syntheses of medical literature)
• CEDIL (Centre of Excellence for Development Impact and Learning)
• Project Evident
• Hubbard Decision Research
• Strategic Decisions Group

Of these, most operate on a fee-for-service model and wouldn't necessarily be able to make good use of individual donations in the, say, $5k-and-under range. However, I believe that Alliance for Useful Evidence and Campbell Collaboration specifically operate on shoestring budgets and are mostly funded by contributed income, so I'd check into those first if you're considering a donation.

FYI, the improving institutional decision-making (IIDM) coordinating group within EA is working on a resource directory that will eventually be able to answer questions like these in greater detail. We'll be posting more about that on the EA Forum later this month.

Disclaimer: I am a co-founder.

The Simon Institute for Longterm Governance. We help international civil servants understand individual and group decision-making processes to foster the metacognition and tool-use required for tackling wicked problems like global catastrophic risks and the representation of future generations.

We have a well-researched approach and direct access to senior levels in most international organizations. Given that we just launched, we have no sense of our effectiveness yet but hope to provide a guesstimate by 2023. 

You can donate to us here.

Institutional decision making

Updated: 2020-12-08

• Quantified Uncertainty Research Institute
• The Good Judgment Project

Note: I document this here: https://causeprioritization.org/Mechanism_and_institution_design (I might not keep this answer up-to-date, so check out the link)

Note: I don't have the impression The Good Judgement Project has room for more funding. I like what the people behind QURI have been doing (I've been following their work). Disclaimer: I was contracted by both groups, and could be again.

Also, documented here: https://causeprioritization.org/Forecasting :

• Replication Markets
• Social Science Prediction
• Metaculus (is for-profit I think)

Note: I don't know if any of those organizations have room for more funding.

I work for the APPG for Future Generations (https://www.appgfuturegenerations.com) in this space. Or impact report is here: https://forum.effectivealtruism.org/posts/AWKk9zjA3BXGmFdQG/appg-on-future-generations-impact-report-raising-the-profile-1 If you wish to donate please get in touch.

The APPG is affiliated with the Center for the Study of Existential Risk (https://www.cser.ac.uk/) which I behind is the best research organisation with content related to longtermism and improving institutional decision making.

More generally I think Transparency Intentional (https://www.transparency.org/en/) and Global Witness (https://www.globalwitness.org/en/) are the dominant charities in the space of reducing government corruption, a key feature of improving institutional decision making. I have not seen any evaluations of them but I'd reject they'd do well.

See also some of the institutions listed in this article (https://forum.effectivealtruism.org/posts/94QtuT4ss3RzrfH8A/improving-institutional-decision-making-a-new-working-group) under the section on "IIDM within and outside of EA"

FHI's Centre for the Governance of AI

someone I know posted the following names on my facebook post for this ea forum post:

• nested minds network
• Verses
• Aeos (soon)

but I am unsure which organization they are reffering to (could not quickly find websites for some of those)
and I could not find an obvious donation form for those 

RadicalXChange

Global coordination

• Global Challenges Foundation
• Good Country
• Together 1st
• The Stimson Center

Note: I document this here: https://causeprioritization.org/Global_coordination (I might not keep this answer up-to-date, so check out the link)

Note: Inclusion in the list doesn't mean endorsement. I love GCF, but I don't have the impression they need more funding. I feel good about the Good Country. I don't know the other 2 well.