The iRig Recorder from IK Media is great for mobile recording. There is a free version as well as a paid, full version. The free version can record and optimize volume, while the full version ($4.99) also includes a waveform editor. Depending on your needs, you may be satisfied with using your device's integrated mic, but the app does support external microphones. Whether you want to record a lecture, dictate a "note to self", or whatever else comes to mind, iRig Recorder is worth a try.
You can read more about the app and the features of both versions here: http://www.ikmultimedia.com/products/irigrecorderandroid/