Please check out our webpage at http://myphilanthropedia.org to familiarize yourself with our color scheme and value proposition (feedback is welcome :).
In terms of what we are looking for, we are fans of elegant simplicity. We want to make the portal as mainstream as possible and really like others that have done the same (twitter, facebook, etc. that have very simple logos).
Thus, our first preference is probably a logo composed of two parts - a nice looking "Philanthropedia" and a corresponding "P" that we can use in documents. For example http://blenderdesigngroup.com/files…ObmE7N.jpg is a nice P with seeds (of knowledge).
Other images that we like, but are more complicated include a tome of knowledge / book, a wallet (our slogan is "Enlighten your wallet"), other elements of wisdom (glasses, owl, staff, etc.). If you take this approach, the benefit is that we could communicate our value with the logo (i.e. knowledge). We think this might get too complicated and so our preference is in the simplistic approach outlined above, but we are open to interesting entries.