I think DomCop can be a good value depending on your needs, but if you are on a budget expireddomains.net is 100% free and great for broad searches, and for more focused searches with metrics (similar to DomCop but free) I use domainresearchlabs.com as they provide lists of SEO domains already vetted for spam and other metrics, kind of a weeded-out version of DomCop without the fee.