(I'm not the grandparent's author, but:) Re-posting the site's contents without attribution may perhaps be a bit iffy, but this exact sorting is what I wanted to see when I was browsing http://ycuniverse.com/ycombinator-companies. The site has this list sorted alphabetically by category title, but it's hard to scan that list to get a feel for the "most popular" categories. It almost feels like clicking something there should make it do that, but instead it just links me to different parts of the huge raw list below (which is super-helpful for someone digging in and doing deeper research but not as useful for someone, such as myself, doing a more cursory glance).
web-based business productivity and collaboration - 21
devices/hardware/components (manufactured goods) - 20
search tech and messaging (contacts/email/sms/text) - 18
travel-related services - 18
employment resources - 16
games and game features - 16
analytics tools/data visualization/and big data - 15
video technology - 14
online music - 13
daily deals/coupons/and price comparison - 12
food discovery/catering/and restaurant technology - 12
photo sharing and social networks - 12
visual design - 12
fashion and clothing - 11
platform as a service - 10
e-tail infrastructure - 9
medical industry - 9
advertising/seo technology - 8
education - 8
event planning (real world) - 8
real estate infrastructure - 8
cloud storage - 7
database technology - 7
logistics/shipping/and warehousing - 7
nonprofit organization - 7
mobile-specific platforms and development tools - 6
parenting resources - 6
screen-sharing and collaborative real-time editors - 6
business communication infrastructure - 5
charity and philanthropy - 5
education/programming - 5
legal services - 5
news/reading/and publishing - 5
scientific research tools - 5
electronics design - 4
physical fitness - 4
programming tools - 4
business human resources - 3
gift card technology - 3
printing solutions - 3
psychometric tools - 3
surveys and forms - 3
user interfaces - 3
agriculture and farming - 2
biotechnology - 2
green energy technology - 2
home improvement - 2
memes and jokes - 2
proximity-based mobile apps - 2
food products - 1