Apache HTTP Server - Apache httpd has been the most popular web server on the Internet since April 1996
draw.io - Online diagramming application
Apache Tomcat - An open source software implementation of the Java Servlet and JavaServer Pages technologies
LucidChart - LucidChart is the missing link in online productivity suites. LucidChart allows users to create, collaborate on, and publish attractive flowcharts and other diagrams from a web browser.
LiteSpeed Web Server - LiteSpeed Web Server (LSWS) is a high-performance Apache drop-in replacement.
PlantUML - PlantUML is an open-source tool that uses simple textual descriptions to draw UML diagrams.