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Avalonia
Develop Desktop, Embedded, Mobile and WebAssembly apps with C# and XAML. The most popular .NET UI client technology
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InfluxDB
Power Real-Time Data Analytics at Scale. Get real-time insights from all types of time series data with InfluxDB. Ingest, query, and analyze billions of data points in real-time with unbounded cardinality.
But you can use Visual Studio Code or JetBrains Rider with Avalonia UI for example: https://avaloniaui.net/
MonoDevelop has been forked into DotDevelop https://github.com/dotdevelop/dotdevelop It can do gtk not WinForms.
NOTE:
The number of mentions on this list indicates mentions on common posts plus user suggested alternatives.
Hence, a higher number means a more popular project.