Mostly looking for programming, helping me visualize the edge cases and what variables I need to use. Therefore, visualizing moving the fields is niceā¦I think. Maybe pen and paper is easier, we'll see.
Downloadable
GoJS
yEd
Online
Gliffy lets you attach to corners with straight lines too.
LucidChart is also pretty good, probably more similar to OmniGraffle.
Google Draw does a decent free job.