* Ex-husband/ex-wife
* Ex-boyfriend/ex-girlfriend
* Ex-employee
* Ex-president
* Ex-soldier
* Exhale (to breathe out)
* Exonerate (to clear of blame)
* Exorbitant (excessively high)
* Exterminate (to destroy completely)
* Exodus (a mass departure)
* Exult (to rejoice greatly)
Let me know if you'd like more examples or have other prefixes in mind!