AppSignal!

Rollbar collects errors that happen in your application, notifies you, and analyzes them so you can debug and fix them. JavaScript, Ruby, Python, PHP, Node.js, Android, Flash, .NET, and Java libraries are available.

Datadog is a monitoring service for IT, Dev & Ops teams who write and run applications at scale, and want to turn the massive amounts of data produced by their apps, tools and services into a unified view of IT infrastructure. Datadog's capabilities are provided on a SaaS-based data analytics platform that enables multiple teams to work collaboratively on infrastructure issues.

Mist.io is an open source software and a freemium service that helps you manage and monitor virtual machines (VMs) across multiple public, private and hybrid clouds using your mobile phone, tablet or laptop.

Comprehensive website and web application monitoring:* simulate a real browser (Firefox) to check performance* services monitoring (FTP(S), DNS, PING, TCP, SSL, SMTP, POP)* user experience monitoring (browser load time per user-agent, response time in various platforms* response time and throughput of each web transaction with graphical representation for quick performance insight* performance stats by ISP* application performance monitoring (monitor Java, .NET and Ruby on Rails web transactions and drill down to offending code and pinpoint root cause of issues from the cloud)* cloud monitoring (Amazon EC2, RDS instances and S3 Bucket)* VMware Monitoring* server Monitoring (Linux, Windows)* On-Premise Poller to monitor intranets* monitor from 3G/4G carriers* DNS server monitoring* SSL certificate expiration monitoring* notifications via SMS, Email, Push Notifications, Twitter and RSS Feeds* mail server, Exchange, FTP monitoring* API, user management* Root Cause Analysis - a comprehensive report based on trace route, ping output, DNS reports and web page screenshots that gives you a conclusion on what caused the issue »

Monitis is a Cloud monitoring and IT analytics service offering Website Monitoring, Server Monitoring, Network Monitoring, Application Monitoring, Cloud Monitoring, and Custom Monitoring.

Uptime monitor for server. Currently fully free (beta) so it is subjected to change. Allows to monitor unlimited servers and websites. With custom CNAME you can create live status pages.

Airbrake collects errors generated by other applications, and aggregates the results for review.

NodeQuery provides a lightweight and easy to use Linux server monitoring service. Get alerts and actively monitor the performance of your servers.

Website & Web Application Monitoring

RapidSpike is an uptime and performance monitoring service for websites and web applications. It simplifies monitoring, alerting and reporting to support your online presence and to ensure a positive user experience. Organisations use it to identify problems, measure performance and improve capacity planning.

SynTraffic provides real-time monitoring for your websites from any geographical region around the world. Smart alerts notify you about important issues as soon as they occur while helping you deal with alert storms gracefully.

God mode in Production code. OverOps tells developers why code crashes and threads freeze. Server debugging doesn't have to suck.