pretty much any 1:18th scale line of toys is interchangable in some way with star wars figures.
I basically stick to joes because of the superior construction and of course the articulation... but I've been known to use marvel figures and other lines too. Pretty much all of hasbro's 3.75" figs are interchangable if you know the right techniques to make them so.
However, there are other toy brands that make 1:18th figures, and there are some really detailed military figures out there that are great matches to the scale.
even some of the other 4" lines work well. I believe there is a 4" line of halo figures, they are slightly over-sized, but when you use the non-humanoid figs the size isn't an issue.