I have KnitGauge, iKnit Ndlszr, and Stitch Minder. All are useful. I haven't tried KnitMinder yet. KnitGauge is great for figuring out how many stitches per inch and determining gauge. Stitch Minder was free and it's a good row counter for one project, except I need to extend the time before the screen goes blank, otherwise it's pushing several buttons just to add one row to the counter.
My favorite is YarnPirateer; not because it's a good app (it's not until they update it with a lot of modifications) but because it has the coolest icon of skull and crossed knitting needles! (And it's free!)