A Simple List of Nostr Signers
Please research what options would work the best for you before choosing.
The Browser Extension:
Flamingo – Nostr extension - Chrome Web Store
A browser extension for Nostr
nos2x - Chrome Web Store
Nostr Signer Extension
AKA Profiles - Chrome Web Store
Nostr Signer Extension supporting multiple public / private key pairs.
Unlock the Keys.Band Chrome Extension
Discover the Nostr Nip07 extension for browsers.

GitHub - haorendashu/nowser: A nostr signing project.
A nostr signing project. Contribute to haorendashu/nowser development by creating an account on GitHub.
The Remote Signer:
Nsec.app - Nostr Key Storage and Remote Signer
GitHub - kind-0/nsecbunkerd: nsecbunker daemon
nsecbunker daemon. Contribute to kind-0/nsecbunkerd development by creating an account on GitHub.
Native Android Signer:
GitHub - greenart7c3/Amber
Contribute to greenart7c3/Amber development by creating an account on GitHub.
NFC and Hardware Signers:
GitHub - yukibtc/keechain: Bitcoin PSBT signer, descriptor exporter and more.
Bitcoin PSBT signer, descriptor exporter and more. - yukibtc/keechain
GitHub - lnbits/nostr-signing-device: Signing device for Nostr
Signing device for Nostr. Contribute to lnbits/nostr-signing-device development by creating an account on GitHub.
GitHub - lnbits/horse: hardware remote nostr event signer with webserial
hardware remote nostr event signer with webserial. Contribute to lnbits/horse development by creating an account on GitHub.
High Security an Recoverability Options:
To start using Nostr will secure, recoverable keys with Promenade:
Create your Nostr account
Create your Nostr account, back it up, and get a Nostr Connect bunker URL in few easy steps!

For existing keys:
Frostr
Thank you to https://nostr.net/ for keeping a thorough list of Nostr apps, clients, and tools!