Perhaps my favorite of the bunch, ArtikPix ($30) is a great way to continue therapy between sessions with a therapist. The app uses flash cards and matching activities — with highly engaging symbols and words — to get children to say words in their trouble areas. In particular, the app's ability to record audio and score answers really helps kids understand if their pronunciation is correct.
The full version of the app comes with all 24 decks with 40 cards each (113 cards in each deck and 1,033 total cards) for the following sounds: th, w, y, h, f, v, ch, sh, k, g, s, z, l, r, s-blends, r-blends, l-blends, p, b, m, n, t, d, and j. The decks can be combined, selected for sound group (e.g., beginning th, er, etc.), and then practiced with flash cards and matching activities.