Basic
Various of these raise security questions, i.e. in CopyQ there is no way of marking passwords on the clipboard as secret unless they come from certain password apps, and that is quite tedious, and presumes you aren't using a command-line password manager, or a browser plugin.
Zazu
App launcher Zazu offers a clipboard manager
Diodon
diodon (ubuntu unity, therefore obsolete).
Shared clipboads
Share clipboard
Share clipboard runs a custom local network service to share across desktop and mobile platforms. It's not clear to me how secure this is but at least it's probaby not transmitting your passwords across the whole internet.
There is even a JS/node version
Ever wondering if it’s possible to copy text from an iOS device and paste it to another iOS device? Or even copy text back and forth between your iOS device and your PC/Mac/Linux computer? Yes, you got the answer, Share Clipboard makes all these possible!
Altcopy
Copy text from your PC to your Smart device's clipboard, and Paste text from your device's clipboard to your PC, instantly! All you need is the PC application, and the Android application on your device. No sign-in required!
Text can be copied over any network. As long as your PC and Smart device both have internet connections, you're good to go. Alt-C uses Google Cloud Messaging with SSL connections to copy the text.
Note google cloud messaging is deprecated whichi is possibly an indication of undermaintenance.
Supports everything apart from iOs.
CopyQ
Cross-platform clipboard manager CopyQ (Cross-platform means desktop, not available for smartphones) offers syncing via file on disk. If you have secure sync this might be attractive to you. It even provides a circuitous way of getting data to smartphones, maybe.
Clipman
A sometimes-recommended windows one I haven't used is clipman
command-line clips
As mentioned elsewhere but collated here.
macOS
cat foo | pbcopy
x11
See linux hack.
The command i xclip. The argument you need to mean that you can paste what
you copied is -selection clipboard.
See stackexchange.
cat foo | xclip -selection clipboard
This is obviously stupidly much typing for your default use case. Define an alias:
I can never remember this half way through a pipe when I need it, so put this in the bash startup file:
alias pbcopy "xclip -selection clipboard"
This is automatic in fish, which per default supports
fish_clipboard_copy
to automagically copy using the best command for your OS.