A small number of sites has blocked my crawler , but that's almost always been my own fault, and happened a few instances when the crawler was misbehaving and actually fetching too aggressively (or repeatedly). In every case just sending an email to the site explaining what happened and humbly asking for a second chance been enough to be allowed back in.
Most website owners don't seem to mind small search engines at all, what they don't want is scrapers that aggressively scrape their entire site 10 times a day, ignoring robots.txt, and being a general nuisance.