Assuming you are interested in embedded development:
I like the olimex boards and the non-olimex boards sold through sparkfun.com. sam7, lpc, etc.
The stellaris boards are very good as well.
You can get a lot of satisfaction and fun out of just getting an old gameboy advance and a supercard sd, or use an nds with an r4ds or cyclods. The nds is a bit more painful for a beginner and the entry cost is three to four times higher than a gba. The gba mini or sp has a backlight which you will appreciate almost immediately over the unlit original gba. And with the gba (or nds) you can get your feet wet with an emulator like vba and not have to buy anything.
The luminary micros are a good start with a nice display and a few other on board gadgets to play with. Be careful with the 811 board as it is really easy to brick as there is a jtag pin on a port with pins you will want to use as generic I/O. On the other hand they are getting stingy with the drivers for the on board peripherals, trying to sell an operating system like experience instead of an embedded experience.
If you want to just write apps on an operating system:
beagleboard or embeddedarm or qemu-arm, its really no different than writing apps for any other linux based machine so the arm factor really isnt there.
