KLayout is a popular and open-source layout viewer and editor for GDS
and OASIS formats. It's used primarily in the field of Electronic
Design Automation (EDA).

GDS and OASIS are file formats for design layouts of integrated
circuits. KLayout allows users to view these complex layouts in a
human-readable form, zooming in and out, panning, and moving around
the design.

In addition to viewing capabilities, KLayout also provides editing
features. Users can manipulate the layout, changing shapes and
patterns, or creating new elements from scratch.

The software provides many more features useful in EDA, including:

Support for different file formats like GDSII, OASIS, DXF, CIF, etc.
Powerful search capabilities to locate specific elements in complex
designs. A macro development environment that lets users automate
tasks. Layout vs. Schematic (LVS) comparison tools. Overall, KLayout
is a highly useful tool for anyone working in electronics design or
fabrication, as it allows the inspection and modification of intricate
IC designs.