The best free alternative to Nagios Log Server is Grafana, which is both free and Open Source. If that doesn't suit you, our users have ranked more than 50 alternatives to Nagios Log Server and many of them is free so hopefully you can find a suitable replacement. Other interesting free alternatives to Nagios Log Server are Prometheus (Free, Open Source), Graylog (Free, Open Source), Open Web Analytics (Free, Open Source) and Logstash (Freemium, Open Source).
Prometheus is an open-source systems monitoring and alerting toolkit originally built at SoundCloud. Since its inception in 2012, many companies and organizations have adopted Prometheus, and the project has a very active developer and user community.
Graylog is an open source syslog implementation that stores your logs in Elasticsearch. It consists of a server and web interface written in Java that accepts your syslog messages via TCP or UDP (or GELF messages) and stores it in the database.
Splunk is software that provides unique visibility across your entire IT infrastructure from one place in real time. Only Splunk enables you to search, report, monitor and analyze streaming and historical data from any source.
Sematext lets you store, index, and search all kinds of logs (server logs, container logs, application logs, mobile app logs…), enabling access to them in one place. Sematext also analyzes your logs for optimal health while detecting anomalies, so you don’t have to.
Rsyslog is an enhanced syslogd supporting, among others, MySQL, PostgreSQL, failover log destinations, syslog/tcp, fine grain output format control, high precision timestamps, queued operations and the ability to filter on any message part.