Knit Counter is a convenient way to store stitch counters and project information for multiple knitting projects.
Each project can have as many counters as you like, and all counters are highly customisable. You can choose the name, the starting value and even link counters together, so one counter increments another. There's also an info screen that lets you note down the pattern name, yarn and needles used, or any other information related to the project.
Other features include a handy frog button that allows you to decrease a counter if you have to rip back rows, and increase/decrease alerts which can be set on specific rows so you never forget an increase or decrease row again.
Details and documentation are available at http://cordlessdog.com/knitcounter
Also available on the iPhone App Store from Cordless Dog:
Knit Counter Lite: All of Knit Counter's great features for free, but limited to a single project.