Forego: to go before, precede. The past tense is forewent, the past participle foregone.
Forgo: to refrain from, to do without; to give up, renounce. The past tense is forwent, the past participle forgone.
Note: Consider the following discussion from Bryan Garner’s The Oxford Dictionary of American Usage and Style (2000):
Although a few apologists argue that these words are interchangeable, they have separate histories. And their meanings are so different that it’s worth preserving the distinction. Forego, as suggested by the prefix, means “to go before.” Forgo means “to do without; pass up voluntarily; waive; renounce.”
Mr. Garner then shows how leading newspapers use incorrect spellings, quoting passages from the Los Angeles Times and the Boston Herald.
Below is an example of a website using forewent when it meant forwent:
[Don Ameche] had a natural gift for acting and got his first professional opportunity when he filled in for a missing lead in the stock theater production of Excess Baggage. After that, he forewent [sic] his law career and became a full-time theatrical actor.