I use to use Pigma, but I found the point on them to grab at the threads of the fabric which made it hard for me to write with. I recently found a make called ZIG in the scrapbooking section that has a ballpoint on them. They're acid-free and archival. They write beautifully. I highly recommend them.

Oh, I also used the FabricMate. The only problem I had with them is you have to be careful when you buy them because of the tips. Some are harder than others. The harder point ones write neater.