I'd definitely recommend Filemaker Pro. Being widely used commercially it's unfortunately not cheap, but for my (PhD) work it's been well worth the cost for its power and simplicity.
I also investigated MySQL, but found it way too involved for the simple single-user database I required. I'd be interested to hear of any other options though. I'm surprised relational databases aren't more heavily used in ecology, and am horrified by how many people trust their data to MS Excel! Andrew
