Inkscape

A full SVG editor — the free counterpart to Illustrator, and native to the web’s vector format.

What it does

Inkscape edits SVG directly: paths, boolean operations, node editing, gradients, clipping and masking, text on a path, and a live XML editor for the underlying document. It exports to PNG, PDF, EPS and optimised SVG.

For site work the useful parts are cleaning up an SVG someone else produced, tracing a bitmap into paths, and Save As → Optimised SVG, which strips editor metadata and cuts file size substantially before an icon goes into assets/.

Notes

It is a GTK application, so it looks like a Linux app on macOS. Functionality is unaffected.
The command line is scriptable, which is handy in a build step:

inkscape --export-type=png --export-width=512 logo.svg
inkscape --export-plain-svg --export-filename=clean.svg messy.svg

Alternative to

Alternative	Type	Trade-off
Adobe Illustrator	Subscription	The industry standard, subscription only
Affinity Designer	Paid	A one-off purchase and a much more polished macOS app
Figma	Freemium	Collaborative and browser-based; better for UI, worse for print
Boxy SVG and Linearity Curve	Freemium	Lighter native SVG editors