A few:
David Drake has a series about an armored mercenary company-Hammer's Slammers-they used a special copper plate that basically stored an ion charge. When activated, POP!, instant particle beam. They had them from handguns to the 20cm main guns on the hover tanks.
Piper apparently figured the 10mm would be a logical firearm for the future, most of his guns were fairly normal by our standards. He and many others wrote before rays and beams and stuff became popular. I'm personally thinking that for unaugmented(read-without power armor or a tank)troops, it'll be a while before guns as we know them become obsolete.
Needle guns are pretty generic. Everything from flechette rounds to coil gun types, I've read at least half a dozen authors that used them in one form or another.
There's the bead guns from the Weber/Ringo
March to the Sea and that series. Essentially coil guns, but very nice.
Foriegn Legions By David Drake had long bows.
Much fun.
Cyantians make use of subspace, using it to store stuff for instant retrieval. Mostly their armor:
http://www.cyantian.net/csafari/archive.php?day=20001013
Among thoughts I've had, are race at peace for so long, they're still using something along the lines of a blackpowder .45-70 for an infantry weapon when boarding another starship. You don't really have to have a technological reason to go to swords and hand-to-hand, if your materials technology keeps up to date with your projectile technology, than guns would never really catch on as anything more than support, much like the bows they replaced. Give enough peace that tactics don't absorb nuclear weapons and such, and you have a nice place to have a war when modern humans break on the scene.