Developed by Agile Partners with Peavey, Ampkit+ is a powerful amp and effects app that's packed with features. You can plug in and use one of the included presets or you can choose to create a preset of your own.
Ampkit+ also allows you to record yourself playing, so you'll never miss capturing a new riff. If you like to play high gain rock or metal, you'll really like this app because it deals really well with feedback, which not all apps do.
One of the problems with using the iPad is that when you want to make an adjustment, you have to pick the device up, make the changes and then put it down—it messes with the creative flow. Ampkit+ fixes that by operating in portrait and landscape modes, so you can put the iPad in its stand where it's always visible.
Ampkit+ is available in free
($19.99) versions. There are also plenty of extras, like new amps and effects that you can purchase from within the app.