Almost all of these are directly related to running my business, but here we go: Shopify, Zendesk, Mailchimp, Highrise, KISSmetrics, Recurly.

Other "stuff" I pay for that's not quite SaaS: Linode, Twilio, ThemeForest.

GitHub - Code repositories

Springloops - Deployment

Pivotal Tracker - Agile planning tool

Podio - Get work done

New Relic - Server/Application performance monitoring

Pingdom - Uptime monitoring

Blame Stella - Speed monitoring

Sendgrid - Email delivery

CloudFront - CDN

Free services:

Groove - Support tickets & chat (groovehq.com)

Trello - Organization

Stripe - Payments

What's your app?

It's not yet live, though it will be at the end of this month or early January.

New Relic - App Metrics, Postmark - Email, Beanstalk - Git Repos/Deployments, CheddarGetter - Billing API, Flow - Tasks, Use Case backlog, Zoho Creator - Awesome for quick internal apps, Recess Mobile - SMS API, great company/owner, Highrise, Basecamp

Has anyone actually developed his own ERP on the cloud? I did, as I evaluated many SaaS accounting software and find none of them suit my needs, and NetSuite, SAP, etc. are too expensive for me.

Github - Code repo

Skype - phone calls

Pivotal Tracker - Agile - Yo baby!

Raven Tools - SEO and reporting

Freshbooks - Billing

Quickbooks online - "accounting"

Balsamiq Online - Collaborative Mockups

and I think my new SaaS that I will be announcing in about 5 days will be well worth the money :-)

Olark, among others. I'm surprised no one has mentioned them.

Uservoice- Feedback/Support

Github

Pivotal Tracker

Airbrake- Error Reporting

S3 if it counts

Harvest for time tracking, skype, last.fm.

Postmarkapp, Nametoolkit (on Mashape)

Github, Spotify, EVE Online

github

hipchat

xero

skype

dropbox

isocket

shopify

rysync.net

newrelic google apps (email) github cotendo (cdn)

rsync.net - backup

Freshbooks

Unfuddle

Evernote

Skype

Skitch (before it joined Evernote)

S3 & Linode (but that's hosting)

Google Apps

Salesforce

Amazon Web Services

Dropbox

Bill.com

Jobvite

Github

Pingdom

Cloudkick

Passpack

Meraki

MS Intune

Helpjuice.com uses

Helpjuice, for the knowledge base, of course

Github

Skype

Dropbox

Olark

VisualWebsiteOptimizer

Stripe - payments

Sendgrid

Linode (switched from Rackspace)

Balsamiq

fogbugz, dropbox, cloudapp

spotify

dropbox

sendgrid

skype

(app engine)