It depends what you need:
| Alternative | Best For | Price |
|---|---|---|
| PastReads | Free Kindle highlight management | Free |
| Matter | Read-later + highlights | Free |
| Raindrop.io | Bookmarks + article saving | Free / $28/yr |
| Glasp | Social highlighting on web | Free |
| Instapaper | Read-later with highlights | Free / $20/yr |
| Daily Brain Bits | Resurface your own Notion/Obsidian notes | Free / $10/mo |
Most Readwise alternatives focus on consumed content. If you want to reinforce your own knowledge base, meaning notes you've written yourself, that's a different category entirely.