Some Free Domain Appraisal Tools
DomainTops Automated appraisal and keyword tool
DomainScore Compare scores between other similar domains
NamePros Appraisal Forum Domainers willing to appraise
DomainValuation Free appraisal service
Estibot Automated appraisal tool
Note that these tools are far from accurate and cannot be used as a replacement for professional appraisal services. However, you can use all these tools to appraise your domain and take the average of all scores as a good measure on the worth of your domains.