Poison Dagger does not drain HP. It is just cursed. A nice weapon in the very beginning.Ymmu's paw is very good one too. Though it has not big damage, it has 15% to stone that is very nice. Also it adds 30% to paralysis resistance - very good too. It can be possessed rather early - you can go to Nyctalinth just after the Orcogre.
Thieves dagger. Good weapon for the thief. Always sold by Blienmeis (in Thieves Guild). But you got to get to Rattkin ruins. It is also possible after the Orkogre.
Vorpal Blade. Sold by Rossarian, but after 131 days of playing and is not guaranteed. Also found 100% in the chest in Rattkin Ruins. Also found in Barlone's treasure, but it is not accessible early. Also found in Brombadeg Cave (in two chests).
By the way Vorpal Blade does slightly more damage than Sword of Hearths, but in other respects it is less powerfull.
If your thief is faerie, then Rod of Sprites is very good weapon.
Has small chance to be dropped by demons (very small indeed). Can be found in Dane Tower (Magna Dane' floor, good chance). and in the brombadeg's cave.
Coil of the Serpent is great weapon. But you need to compensate its regen -1 and it is located on the Magna Dane's floor of the Dane Tower.
I did not considered extended/thrown/long weapons here, only close ones for frontal thief.
Also I believe Thieves Dagger is better than Sword of Hearths. TG does 7.5 damage in average, while SoH does 8. No much difference, heh? but TG has 5% Kia and poison that could give additional 2 damage per round here and there, SoH only has 2% kia. TG is also lighter than SoH. so...