Gnu C Compiler (GCC)
GCC is the user-supported compiler developed by the Free Software Foundation. Many people have worked on adapting it for PalmPilot development. Along with the GCC compiler itself, other included tools combine to create a complete development environment.

GCC has several advantages. The price is tough to beat - it's free. The tool set is quite good; many people have developed applications using it. It's user-supported - as new PalmPilots come out, any changes in the operating system (and changes in the APIs) need to be rolled into GCC and its support files. Despite the lack of profit motive, there are quite a few people who've worked on the PalmPilot GCC; it's reasonable to believe they'll continue this work - it's quite possible they'll have new-API support before Metrowerks does. On the other hand, if no one does do this support work, it will lag behind. This is the problem with Pila; it languished for most of the past year, only recently being ported to Linux with some enhancements.

Most Unix and Linux users will already be familiar with GCC, and will find it quite comfortable. Windows users will likely find it unnatural at first. It has a strong "Unix-ish" feel to it; to be expected given its Unix background.

CodeWarrior
Metrowerks' CodeWarrior, originally only Macintosh-hosted, is now available for Windows. Given its background, it has a strong Macintosh feel to it; Macintosh developers will be quite comfortable, but Windows users will find things quite odd - menus change depending on the active window, buttons and toolbars are often strange, common key-combinations don't work, etc. The Windows version is not quite as complete as the Macintosh version - as well as lacking Gremlins, the Windows resource constructor lacks some bitmap tools. Release 4 is due out as I write this; it's an improvement over Release 3, but there are still a number of rough edges for Windows-hosted development. Nothing insurmountable, but a number of inconveniences. The Windows version lags behind the Macintosh version in several features.

Overall it's a reasonably solid tool set from a company that responds to its customers - even to the extent of adding a #pragma to handle one of my own complaints. If your preference is C++, Release 4 improves support for C++.

CodeWarrior is $369; academic versions are available for $119. Metrowerks also bundles CodeWarrior with either a PalmPilot Personal or Professional; if you need another (or a first!) PalmPilot, this is an economical solution. Or you can do as I did, and sell the spare PalmPilot, reducing the overall cost of the compiler.

Pila and ASDK
Pila and the ASDK (Alternative Software Development Kit) are a set of tools for developing in assembly language. Some of the tools are the same as used with GCC. While this is a personal favorite of mine - well-optimized assembly code is tough to beat for size and speed - it's not an environment I'd recommend easily. All the PalmPilot documentation is C-centric, so you're constantly mentally converting to and from C issues - API arguments and return values, structures, etc. Some built-in macros help with this, but it takes a while to get used to it. Unless you're very comfortable in assembler you'll run into a number of problems which the compilers help to insulate you from - word access to odd boundaries, incorrect API arguments, failing to save and restore registers, etc.