Important crawlers?

Google:
Crawler Description
Googlebot Crawls web pages (it’s the most important of the bunch)
Googlebot-Mobile Crawls pages specifically designed for mobile devices
Googlebot-Image Crawls images for inclusion in image search results
Mediapartners-Google Crawls AdSense content
AdsBot-Google Crawls AdWords landing pages to measure their quality

Yahoo!:
Crawler Description
Slurp Crawls web pages
Yahoo-MMAudVid Crawls video files
Yahoo-MMCrawler Crawls images

Bing:
Crawler Description
MSNBot Crawls web pages
MSNBot-Media Crawls media files
MSNBot-News Crawls news feeds