Hi Ric, I attend as many New Media Consortium (www.nmc.org) conferences and events as I can they are a nonprofit associated with the educational sectors.
I also keep up with the Metanomics sessions (www.metanomics.net) which isn't a nonprofit I don't think however business and economics discussions regarding SL/virtual worlds are very informative.
Transitions ~ A Place for Dreams (www.aplacefordreams.com) is a joint initiative of 5 nonprofits including Bridges for Women, who met at the Nonprofit Commons.
I've have had discussions with a few of the nonprofit consulting for fundraising or grant writing which I found very helpful to take back to Bridges.