Kaywa Reader is one of the most impressive commercial readers. Notably, it can recognize codes in a video stream from the phone's camera. The user never needs to press a button to take a picture.
Libdmtx* is an open source (LGPL) library for decoding Datamatrix codes. It is designed to be highly portable, and it is written in C. It was recently made available again after a year-long break.
ZXing* is an open source (Apache 2.0) application recently released by Google that aims to "support decoding of QR Codes, Data Matrix, and the UPC family of 1D barcodes."