I wrote FS1541 in 1996 for the Catweasel controller (but it also works on disk image files, you'll need fmsdisk.device in that case). It gives you read and write support for the disk format of the legendary C-64 home computer. The archive is available from your favourite Aminet site as misc/emu/FS1541-13.lha (64K) (complete source code in C is included). Here's the previous version, FS1541-12, if you have any problems.
Update: (23-Oct-2007) Helmut Schoettner has added some features and fixes over the years; here you can find his version - thanks, Helmut!
I once wanted to extend FS1541 to handle 1571 and 1581 disks as well, but since simply hacking it into the existing code would have been rather messy, I decided to rewrite everything from the beginning. libcbmfs was going to be a collection of routines to handle various disk formats used in the old Commodore home computers such as the C64 and C128. I'm not doing much work on it any longer (reading files from disk images works, not much more), but planned was support of the 1541, 1571 and 1581 formats, including writing and automatic partition handling. Here is the code I've written so far (in August 1998).
As for Linux, there's two relevant pages you might want to have a look at: Dan's 8-bit Commodore Information page; Dan has written a Linux filesystem (also called cbmfs), and David Weinehall's page; David has taken Dan's code in September 1999, added some features and updated it for the 2.2 kernels. No write support, though.