Below is a compilation of lists from a few recent sources:

NetworkWorld – May 21, 2014
(Alphabetical)

• Amazon
• AT&T
• Axeda
• Cisco
• GE
• Google
• IBM
• Intel
• Microsoft
• Oracle
• Salesforce.com

http://www.networkworld.com/article/2287045/wireless/153629-10-most-powerful-Internet-of-Things-companies.html Retrieved 26 June 2014.

Fast Company – February 10, 2014

1. Nest (Google)
2. Philips
3. Quirky
4. Jabone
5. SmartThings
6. Withings
7. Belkin
8. Intel
9. LogMeIn
10. R/GA

and from the comments section

• Ayla Networks
• Brivo Labs
• Carriots
• Sen.se

http://www.fastcompany.com/most-innovative-companies/2014/industry/the-internet-of-things Retrieved 26 June 2014.  Archive (PDF)

Internet of Things Conference (UK)

List of 2013 Companies (no apparent order)

• Telefonica
• Deutsche Telekom AG
• Jasper Wireless
• Ericsson
• Microsoft
• IBM
• ARM
• Machina Research
• Everything Everywhere
• Libelium
• Orange
• Wind River
• Gemalto
• Neul
• Pyramid Research
• Sony Mobile
• Inmarsat
• Morgan Stanley
• Cumulocity
• Amdocs
• Net4Things

http://iotinternetofthingsconference.com/companies/ Retrieved 26 June 2014.  Archive (PDF)