The following are some web-based tools I have found useful for editing when I don't have access to software.

• Image editor: Photopea (wiki article), a highly featured browser-based editor. Not open source, sadly, but is damned good.
• IPOL.im: lots of useful and interesting demos, everything from deblur to geometry correction.
• RawWorks, a browser-based RAW editor.
• G'MIC Online -- port of G'MIC, sadly in the cloud, but FOSS. Good for signal processing.
• FisheyeGL: browser-based de-fisheye program. Not über-precise, but can substantially improve distorted images.
• ImageJ Port of w:en:ImageJ to the browser. Very technical, but it is a powerful tool if you know what you're doing (I don't).
• Video editing: Wide.video, an ffmpeg-based editor. It can also create SubRip (VVT, but those can be converted into SRT easily) files using the GPU at a reasonable speed.
• Audio editing: Audiomass, I mostly use it for its noise reduction features. There's also a port of the software Audacity, but the UI is kinda off-putting.
• Misc: FFmpeg. The command-line interface is fairly powerful, and ChatGPT is helpful. There are also various GUIs out there. All the FFMPEG WASMs are rather sluggish (for video especially).