Estibot.com - Good for data, but ignore the final valuation price
NameBio.com - Similar Sales
TDVR.com - LLLL.com Similar Sales
NameDroppers.com - See what other domains are regged with keywords
Google Keyword Tool / Google Trends
A while back I developed a list of over 65 different checks to do a domain background check on any domain. Had that process automated via DNProtect a few years ago when that service was live. Those checks use most all of the tools mentioned--but there are a few new ones I'll check out.
But for deciding whether or not to renew a domain I own, it's fairly limited: traffic to it the past year, whether I've had offers (automatic renewal if that's the case), and the number of whois lookups on that domain. My preferred registrar(s) give me that data (how many have looked up the whois for the domain).