I've been assembling a list of these lately for a book that I'm working on. (http://startingandsustaining.com) Some of the categories are fairly loose as some apps don't fit nicely into categorical buckets, but hopefully this is a helpful list.

--Browser/Email Testing

BrowserStack (http://www.browserstack.com)

Litmus (http://litmus.com)

--Bug/Issue Tracking

BugHerd (http://bugherd.com)

Lighthouse (http://lighthouseapp.com)

Sifter (http://sifterapp.com) (Disclaimer: I built this.)

--Planning & Project Management

Sprintly (http://sprint.ly)

Podio (https://podio.com)

Flow (http://www.getflow.com)

Interstate (http://interstateapp.com)

Basecamp (http://basecamp.com)

Apollo (http://www.apollohq.com)

Pivotal (http://www.pivotaltracker.com)

Asana (http://www.asana.com)

Trello (https://trello.com)

Blossom (https://www.blossom.io)

Trajectory (https://www.apptrajectory.com)

--Business & Traffic Analytics

KissMetrics (http://kissmetrics.com)

MixPanel (http://mixpanel.com)

DigMyData (http://digmydata.com)

--Continuous Integration / Code Quality

Travis (https://travis-ci.org)

Circle (http://circleci.com)

CodeClimate (http://codeclimate.com)

Sempaphore (https://semaphoreapp.com)

--Dashboards

Ducksboard (http://ducksboard.com)

Geckoboard (http://www.geckoboard.com)

Instrumental (https://instrumentalapp.com)

--Error/Exception Handling

Sentry (https://getsentry.com)

Coalmine (https://www.getcoalmine.com)

HoneyBadger (https://www.honeybadger.io)

BugSnag (https://bugsnag.com)

Raygun (http://raygun.io)

--Log Monitoring

Loggly (http://loggly.com)

Papertrail (https://papertrailapp.com)

LogEntries (https://logentries.com)

--Billing & Payment Processing

Braintree (https://www.braintreepayments.com)

Stripe (http://stripe.com)

Pin (http://pin.net.au)

PayMill (http://paymill.com)

Recurly (http://recurly.com)

Chargify (http://chargify.com)

Spreedly (http://spreedly.com)

Spreedly Core (https://core.spreedly.com)

--Support/Help Desks

Desk (http://desk.com)

HelpScout (http://helpscout.net)

ZenDesk (http://zendesk.com)

Groove (http://groovehq.com)

Intercom (http://intercom.io)

Tender (http://tenderapp.com)

--Transactional Email

Postmark (https://postmarkapp.com)

Mandril (http://mandrill.com)

MailGun (http://www.mailgun.com)

SendGrid (http://sendgrid.com)

CloudSMTP (http://www.cloudsmtp.com)

CritSend (http://www.critsend.com)

Postage (http://postageapp.com)

--Email Collection/Landing Page Apps

Launchrock (http://launchrock.com)

Unbounce (http://unbounce.com)

KickoffLabs (http://www.kickofflabs.com)

Launch Effect (http://launcheffectapp.com)

Prefinery (https://www.prefinery.com)

LaunchGator (http://launch.deskgator.com)