DAVID DUNHAM  This works under Snow Leopard (10.6), and also with a small wired keyboard (i.e. you don't need a MacBook). I only use it to remap the Enter key to where my muscle memory knows it is, but I'd be cursing Apple without it.  
(Version 5.1)

SPLUNK23  Starting with version 3.1 this program has the ability to enable the directional and Ins/Del keys on the numeric keypad (of a PC-type keyboard), as well as PC-style Home and End keys. So if you are using a PC-style keyboard (for example, with a Mac Mini) and when working on a PC you never set NumLock on the numeric keypad, now you can have the operation you are accustomed to. Personally this was my #1 irritation with the Mac Mini (normally the numeric keypad generates numbers only, and the NumLock key is disabled), and this software restored the operation I was familiar with. The program has worked flawlessly for me, but you do need to make sure that no other keyboard modification programs are installed, or you might get conflicts that render your keyboard unusable until you uninstall one or the other. So if you have DoubleCommand or some similar keyboard mod software installed, make sure you uninstall it prior to installing this software.  
(Version 3.2)

PHASEIV  KeyRemap4MacBook is a very good (japanese) alternative for Doublecommand users.

KeyRemap4MacBook is able to remap functionality for all modifier keys, remap keys to Emacs-mode

and SandS-mode and provides EISUU and KANA options for japanese keyboards.

There are several other cool settings, and if used with the PCKeyboardHack extension you can use

F3, F4 and F5 for hardware volume control while F8, F9 etc. keep working for Expose and Spaces.

Minor downside is that the remap setting(s) need to be applied through the terminal, but once its set

the program performs very well and stable. (version 2.3.1, MBP 2.4 SR)

Check the developer's site for an overview of the remap options and how to apply them.  
(Version 2.3)