Today, I set up a GameBoy DMG development toolchain on Raspberry Pi.  Here's how I did it.

-----------------------------------------------------------
+-------------+
| Tile Editor |--.
+-------------+  |
                 |
+-------------+  |    +-------------+       +-------------+
| Example ASM |--+--> |  Assembler  |--+--> | GB-Emulator |
+-------------+  |    +-------------+  |    +-------------+
                 |                     |
+-------------+  |                     |    +-------------+
| Your ASM :) |--'                     '--> | Flash Cart. |
+-------------+                             +-------------+
-----------------------------------------------------------

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Tile Editor
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BogoTile.
http://github.com/PeterFidelman/BogoTile


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Example ASM
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Use the example files from WLA-DX.
http://www.villehelin.com/wla.html - scroll down to "Example Sourcefiles".


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Template ASM
------------

https://github.com/PeterFidelman/ASMTemplates


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Assembler
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WLA-DX.
https://github.com/vhelin/wla-dx


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GB-Emulator
-----------

no$gmb.
http://problemkaputt.de/gmb.htm

Run it in dosbox - 'sudo apt-get install dosbox'.
It doesn't quite run at full speed, but it gets the job done.


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Flash Cartridge
---------------

emsflasher.
http://lacklustre.net/projects/ems-flasher

Use this with an EMS flash cart to deploy code to your real GameBoy.


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Useful HOWTOs
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The PanDocs.
http://bgb.bircd.org/pandocs.htm

GameBoy Genius and friends.
http://gg8.se