I only use one "i" app that is not one of Apple's iApps that come standard on Macs:
iBank
Also, I think Mail and Preview are technically considered iApps, even though their names don't begin with an "i". Garageband is probably considered an iApp as well (it comes as part of iLife, doesn't it?)
*edit* Thinking about it, I think any app written by Apple could be considered an iApp.
*2nd edit* I've always wondered why Apple never got any sort of protection to naming apps a lowercase "i" + capital letter of whatever it is called (perhaps they tried and were not able to). It seems to me that when a 3rd party names their app in this fashion, it gives the false impression that their app is somehow endorsed by Apple.
*3rd edit* I realized I also use iGetter