I refuse any contract where my employer claims ownership of code I wrote in my own time on my own machine that has nothing to do with the work I do for the company. Very few contracts have contained such a clause, and I believe at least one removed such a clause when I requested it.

These days I'm self-employed and renting myself out to large companies, and this has never been an issue. They only care about the work I do for them, not about what I do in private.