I've got one here that's quite powerful:
http://www.white-flame.com/wfdis/ It requires HTML5 & Javascript enabled, but will run anywhere that runs. You can even download & run a local copy; it's not technically "web based", as it runs completely in local javascript.
It's currently Commodore 64 specific, but if you have a binary, prepending the 2-byte load address to the file will make it work as a "prg" file.
Features:
- Tracing 6502 disassembler
- CBM BASIC detokenization, finding SYS addresses and disassembling from there
- Very slow emulator for running relocation or decompression code, allowing you to disassemble further from that
- Understands .prg and .sid formats, as well as .d64 images containing them
- Edit formatting (indentation, blank lines), and comments
- Create or rename labels
- Built-in C64 ROM & I/O labels
- View areas as some C64 graphics types
This is technically just the front-end to my AI-driven disassembler back end, but I am also keeping it fully runnable client-side with just the rudimentary tracing disassembler.
Big caveat: no saving/export yet. Saving & loading the disassembler state does work, but there's no UI hook for it yet. Still, you can explore the workings of a file, and copy/paste out if you're desparate.
Small caveats: No "illegal" opcodes yet. Manually switching things from gfx back to binary sometimes doesn't register quite right.
To get more features in, just bug me.