What do you look for when swapping links? Look for sites that are in your industry, share a common audience but aren't your direct competitors, and are sites you would like to exchange website traffic with.

I don't believe they have a negative effect to SEO unless you participate in free-for-all link pages, link farms, are on pages unrelated to your own, massively appear within a short period of time, or anything else that isn't honestly done because you believe the other site is of a high enough quality that you want to associate yourself with them.