The oldest, and probably the most reliable of the net-based surveys can be
Their method of attracting participants uses banner
ads on high-traffic sites to make sure they get to novice
users, and they have ways to prevent multiple submissions.
(if you have ever taken the full GATECH survey, that in itself is
enough to discourage submitting again).
They split users by various demographics - it's boringly
academic - but it is also the longest-running survey and the least
likely to be biased.
Specific results of interest to site promoters: the % of people
who find things via links from other web pages was EQUAL to the number
of people who use search engines. Multiple answers were allowed, but
it's clear that cultivating links pays off.