HOSTING
Rails Hosting: EngineYard
LAMP Hosting: HostGator
File Hosting: AWS
Video Hosting: Wistia
Code Hosting: GitHub
COLLABORATION
Email/Calendar/Docs: Google Apps
Project Management: Basecamp
Group Chat: Campfire
Wiki: Backpack
COMMUNICATION
Email Deliverability: SendGrid
Phone Magic: Twilio
Phone System: OpenVBX
MONITORING
Error Monitoring: Airbrake
App Monitoring: New Relic
Server Monitoring: Pingdom
ACCOUNTING
Receipts: Shoeboxed
Income: Freshbooks
Accounting: Outright
OTHER
Web Fonts: fonts.com
Web Fonts: Typekit
eSignatures: DocuSign
Forms: Wufoo
Advertising: Google AdWords
Our favorite products are probably Outright (dead simple), Twilio (enables tons of phone system magic), and AdWords (flexible, powerful, profitable). The two products we're actively working to replace are Backpack (with Workflowy or something similar) and Shoeboxed (we have yet to find an alternative). And we've outgrown Wufoo, but owe them much more than we've ever paid them in monthly charges.