Cross Assemblers

AS80 by Frank A. Vorstenbosch
Pasmo by Julian Albo
SjASMPlus by Aprisobal
TASM by Speech Technology Inc.
The E-Z80 Way by Fraser Ross (you'll need the Borland Libraries to make it work)
tniASM by The New Image
Z80-ASM by Brainsoft
Z80ASM from Z88DK package by Interlogic (a C compiler is also available)
Z80v4 by Nick Fleming
ZMAC by Bruce Norskog and others

(Yellow highlighted assemblers are distributed as source code and can therefore be used under Linux or any other operating system)

You'll also need this linker for The E-Z80 Way: IntelObject2TAP, Z80v4 will produce a .SNA file and for the rest of assemblers, you can produce binary files and convert them to TAP using BIN2TAP (source here), or using BIN2CODE (source here), that doesn't produce any BASIC header. Pasmo can also produce TAP files with or without basic header without the need of these programs.

Z80 Information

Zophar's Domain: Z80 Technical Documents

More Spectrum graphic editors

TommyGun by Tony Thompson: Complete development system for Spectrum, Amstrad and Commodore, including image edition, tiles & maps design and code edition. It can open .SEV files created by SevenuP.
ZX Paintbrush by Claus Jahn: The screen editor of ZX-Modules. An excellent editor with lots of tools.
BMP2SCR Pro & BMP2SCR Exp by Leszek Chmielewski (LCD): Its main purporse is the conversion to spectrum graphic formats, but the newest versions also have the edition functionality, and can even create sprites and tiles for games.
Spectrum Graphics Editor by Richard Jordan: Uses memory snapshots to find & modify graphics in speccy games, directly on memory. Perhaps it's possible to create new graphics from scratch, but you still need to provide a snapshot first.
ZX Spectrum Graphics Editor by Alexey V.Susekov: Only for screens, can use several russian special formats (as used in demos & games for Pentagon).
SCR-EDitor by Alexander Shabarshin (aka Shaos): Only for screens, and I would reccomend it only for small modifications, because edition is done char by char.