Item description:

The FASMARM package is a free ARM cross-assembler add-on for FASM
Runs under Win32 (9x/NT/2K/XP/Vista/7) and LINUX.
Plus a linkable object file for UNIX/LibC.
FASMARM currently supports the full range of ARM processors and coprocessors.

If you want to compile FASMARM yourself you will also need to download FASM

Tested for the gba by krom, who also mentions:
FASMARM is much more stable than the goldroad ARM assembler...
FASMARM will not currently recognize any of the ARM pseudo-instructions like LDR Rd,=
You will need to set up the gba's literal pool yourself, or use a macro to load any 32-bit numeric value into a register:

macro imm32 reg,immediate {
mov reg,immediate and $FF
orr reg,immediate and $FF00
orr reg,immediate and $FF0000
orr reg,immediate and $FF000000
}
Use imm32 Rd,$12345678 in your code etc...

Everything else is perfect in this assembler which includes a full GUI for you to edit your source code. This is easily the best ARM assembler I have used, also the developer revolution has been updating it on a regular basis.

The download includes both the Windows & Linux binaries, including the full source code.