Here's a list of all the good off-hand weapons. They're all fast enough to get your two attacks, and they're all in the top ranks of accuracy. I'll list rough estimate of (Accuracy, Damage) where 1 is the best and 19 is the worst. These are all based on an old-as-dirt comparison list done by another player. It's good enough to help you decide which weapon is going to work best for you, but none of us can see the real numbers, and he himself said it was a little sketchy in parts.
Kama - the least accurate, most damaging, most rare off-hand. (5, 12)
Wakizashi - only slightly less damaging and slightly more accurate. The only problem is that there are almost no energy/adamantine wakis, and only like one mithril. (4, 13)
Epee - probably the most common and sensible off-hand weapon. Great accuracy, enough damage to justify holding the darn thing in your hand. (2, 14)
Rapier - epee, but less accurate. Some people will tell you it's stupid to use one of these when you could use an epee, but it's very easy to find something like an energy rapier. Honestly, I'd prefer an 8/8 energy rapier to a 4/4 tanso epee. If you have access to the best epees, use those instead. (3, 14)
Sai - the most accurate weapon in the game, with little to no damage at all. It's very popular among swashies who have to tank a lot, since it drastically lowers your chances of getting hit...ever. Those swashies have someone else doing damage instead of them, though, so if you're trying to kill something you might want to go to an epee or something. (1, 17)
There have been a few combinations using other off-hands, but none of them have the same combination of speed and accuracy. People have dual-wielded a certain special kind of straight dagger, for instance, and I think I've seen a swashie with two sickles, but in general you should go with the above choices.
Good main hands, same roughly estimated list.
No-dachi - this is almost exclusively a two-handed weapon, but there are a few one-handers wandering around out there, and if you get the chance you should snap one up. They HURT. (6, 5)
Khopesh - even after it got wimped it's still good. Probably the best main hand after no-dachi. (7,
Katana - nice and accurate. Not so damaging. (4, 10)
Straight Dagger - a slightly worse katana but there are some really cool ones out there that make it worth it sometimes. (5, 10)
Cutlass - Sucks for everyone else, but for a swashie it makes a passable main-hand. By "passable" I mean "get something else quickly." (5, 11)
Main hands I don't recommend but you could use in a pinch.
Longsword - not accurate enough. I don't think you'd last long. (9, 6)
Khukri - same deal. (9, 9)
Falchion - same deal. (10,
Finally, you could always dual-wield two off-hand weapons for lots and lots of accuracy. For example, dual-wielding kamas is awesome.
[Viltrax edit: off-topic posts removed]