Yep homegate.ch seems to be the biggest, although we found our place on www.immoscout24.ch.

One time we were viewing a place and another person who was there viewing recommended http://www.immomailing.ch/. Apparently you pay about 40 CHF and you get emailed places that aren't advertised anywhere else.

In the end we didn't use it so I really don't know how if it's any good, but the person who recommended it to us said it was really good.