There aren't many other options that I've heard of, partly because the Topeak ones do the job well, and partly because they're the main "accessory brand" that's seen in most bike shops thanks to the fact that every shop probably has access to the same distributor.

I added the pannier sides to my beam rack so I could use my panniers that normally ride on my other bike's regular rack. I dig the slide-track design, too; I even got the basket for occasional use.