Handheld Basic - VB type language that doesn't need a run time. Download the IDE and sample projects. - http://www.handheld-basic.com/
HotPaw Software - Home of the HotPaw Basic programming environment for PalmOS. - http://www.hotpaw.com/rhn/hotpaw/
cbasPad by Ron Nicholson - A tiny interpreter for subset of BASIC that runs on the PalmPilot. Includes floating point support, trig and log functions. Can play sounds, do simple graphics, and has serial port support. Freeware. - http://www.nicholson.com/rhn/pilot.html