Software/Webapp for creating computer science diagrams

What are some good programs or web-based applications that can be used (preferably, but not necessarily, for free) to create diagrams for computer science articles or dissertations?

Particularly, I'm looking to represent data structures such as stacks, linked lists, trees, etc..

Any ideas?

How can I make a diagram online for free?

diagrams.net (formerly draw.io) is free online diagram software. You can use it as a flowchart maker, network diagram software, to create UML online, as an ER diagram tool, to design database schema, to build BPMN online, as a circuit diagram maker, and more. draw.io can import .vsdx, Gliffy™ and Lucidchart™ files .

Gliffy should work for you. Many of the student's in my class used it and it worked out great.
Microsoft Visio: Not web based, but quite powerful
Dia: Open source diagramming tool