From Forum Nokia Wiki

• ECompXL Epoc Compressed eXecutable Loader

ECompXL is a development tool that allows you to compress applications (.app) and executables (.exe) for Symbian OS 7. Applications and executables built with ECompXL run as any other on a target device but, when started, will first decompress themselves on-the-fly before their actual execution. ECompXL uses the standard gzip utility for compression and the zlib library for decompression; a port of zlib is part of the standard Symbian OS 7 SDK.

ECompXL is released under a MIT-style license, see the file copying for more information.