6502/65C02/R65C02/W65C02/65CE02/65816/DTV/65EL02 Turbo Assembler

Key features:

Open source, mostly portable C with minimal dependencies
Familiar syntax to Omicron TASS and TASM.
Supports 6502, 65C02, R65C02, W65C02, 65CE02, 65816, DTV, 65EL02
Arbitrary-precision integers and bitstrings, double precision floating point nos
Character and byte strings, array arithmetic
Handles UTF-8, UTF-16 and 8 bit RAW encoded source files, unicode strings
Supports Unicode identifiers with case folding and compatibility normalization
Built-in "linker" with section support
CPU or flat address space for creating huge binaries (e.g. cartridges)
Conditional compilation, macros, struct/union structures, scopes.

WWW: http://tass64.sourceforge.net/