Japanese developer Mitsu Toda has released an update recently to his GSFinder+ file browser. GSFinder+ includes features designed to not only replace Windows (CE) Explorer, but to add additional functionality and features. The main one being split screen tree view.
Other features of note are Zip and LHA support, Network driver mounting and dismounting, network browsing, file searching, association management and more.

GSFinder+ 1.04e is designed to run under most H/PC's with ARM, MIPS, SH3, SH4 processors - including CE .net and the Sigmarion III.