Is there a concern that volunteers could manipulate results through their crawler?
You already mentioned distributed search engines have their own set of issues. I'm wondering if a simple centralised non-profit fund a la wikipedia could work better to fund crawling without these concerns. One anecdote: Personally I would not install a crawler extensions, not because I don't want to help, but because my internet connection is pitifully slow. I'd rather donate a small sum that would go way further in a datacenter... although I realise the broader community might be the other way around.
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Unless, the crawler was clever enough to merely feed off the sites i'm already visiting and use minimal upload bandwidth. The only concern then would be privacy. oh the irony, but trust goes a long way.