From the ASHA Leader, 10/11/11, article "Apps: An Emerging Tool for SLPs" by Jessica Gosnell:
Receptive Language Apps (prices range from$.99 to $8.99) -- Speech with Milo: Sequencing; iPractice Verbs; Speech with Milo: Prepositions; ABA Receptive ID; Understanding Inferences; Splingo's Language Universe; House of Learning; Preposition Remix; & Name That Category

Expressive Language & Speech Production Apps (prices range from free to $29.99) -- PhonoPix; ArtikPix; Speech Trainer 3D; Smarty Speech; Speech Pairs; SLP Minimal Pairs; Story Time!; 60 Story Starters; & Language Builder

Augmentative Communication Apps (prices range from $29.99 to $189.99) -- Proloquo2go; MyTalkTools Mobile; Assistive Cht; AAC Speech Buddy; Tap Speak Sequence; Grade; TouchChat-AAC; Predictable & Sounding Board