It depends heavily on what you are looking to do, I currently really like the stm32f4 discovery. About $20. Hard to call it a microcontroller with the amount of flash and ram, two caches, crypto, floating point, upt 168MHz, supports external memory, etc. (This eval board does not have external memory, 1mb of flash and 128K of ram in one linear chunk, there are other chunks of ram I think).
You dont or dont necessarily get jtag access to it though, the newer microcontrollers from arm are using swd, a serial jtag which needs more time to get all the popular tools supporting.
the omaps are ti and boards like the beagleboard dont use the stock arm pinout. there are a lot of little things like that make the beagleboard itself more painful than it should be. I am eager to see what the raspberri pi folks did.
I like my devkit8000 board from embest much much better than my beagleboard. like the hawkboard better than the beagleboard as well. (btw not difficult at all to do your own thing, non-linux, with a beagleboard).
You might look at embeddedarm.com, being arm7 and arm9 based they are going to offer the traditional arm jtag. They may actually use external memory.
I was also pleased with the open-rd board, cortex-a core but from marvell instead of ti, and faster. disappointed with the plug computer though, avoid it.
The new xilinx chips use an ARM core instead of power-pc, do whatever you want with that and the eval/devel boards probably have flash and ram and lots of other things, but pricy, and can be a pain to get running.
I was not pleased with my openpandora, nor always innovating (built almost directly around the beagle), dont waste your money there. if anything get a nook color and root it or use a gba or nds which are really easy to develop for.
there is also the pandaboard, I dont have one, didnt know it was sub $200, I may have to get one, will probably like it better than the beagle as well.
there are a number of arduino sized boards, the armmite pro, the maple and family, the fez-panda (not to be confused with the pandaboard.org panda board), etc. They cannot compete with the stm32f4 discovery other than form factor if you really really need to put arduino shields on their backs. the armmite pro and fez-panda (once unlocked) have real arm jtag though.
The mbed is good if you really like lpc and hate st for example, costs two to three times as much but is probably my other favorite board after the stm32f4 discovery.