Website Performance Monitoring

• Google PageSpeed Insights: Analyzes the content of a web page and provides suggestions to make it faster.
• GTmetrix: Provides insights on how well your site loads and offers actionable recommendations.
• Pingdom: Monitors website uptime, downtime, and performance.
• WebPageTest: Runs free website speed tests from multiple locations around the globe.

SEO and Website Analysis

• Google Search Console: Offers tools and reports to help measure your site's search traffic and performance.
• MozBar: A browser extension that provides instant metrics for any page or SERP.
• Ubersuggest: Provides insights into keywords, backlinks, and traffic.
• SEMrush: Offers a limited free plan to analyze website SEO, paid advertising, and more.

Website Security Monitoring

• Sucuri SiteCheck: Scans your website for malware, vulnerabilities, and other security issues.
• Quttera: A free malware scanner tool that checks websites for known malicious content.
• SSL Labs: Tests the strength and configuration of your website's SSL/TLS.

User Experience and Behavior

• Hotjar: Offers heatmaps, session recordings, and surveys to understand user behavior (free with limitations).
• Google Analytics: Provides detailed statistics and analytics of web traffic and is a fundamental tool for marketing.

Mobile Responsiveness

• Google Mobile-Friendly Test: Checks how easily a visitor can use your page on a mobile device.
• Responsive Design Checker: Allows you to view any URL in various screen resolutions.

Network Monitoring

• Wireshark: Widely-used network protocol analyzer.
• Nagios: Offers complete monitoring of desktop and server operating systems.
• PRTG Network Monitor: Monitors network devices, bandwidth, servers, applications, and more (free up to 100 sensors).
• Zabbix: Open-source solution for network and application monitoring.

Server Monitoring

• Monit: Small open-source utility for managing and monitoring Unix systems.
• Netdata: Provides real-time health monitoring and performance troubleshooting for systems and applications.

Content and Accessibility

• WAVE: Web accessibility evaluation tool.
• Broken Link Checker: Scans web pages and provides a report of broken links.

Domain and IP Analysis

• Whois Lookup: Provides domain ownership and DNS information.
• MX Toolbox: Checks your DNS and mail servers for common issues.

API Monitoring

• Postman: Popular tool for API testing with monitoring features.
• Runscope: Monitors APIs for uptime, performance, and data validation.

Load Testing

• Loader.io: Free load testing tool by SendGrid Labs.
• Locust: An open-source load testing tool.

The free versions of these tools may have limitations and some tools offer premium features at a cost. Find the free versions that work best for you. After becoming facile you can determine if you need more horsepower from a paid version.