I agree with Insidious that they are primarily useful for leveling alts. Original set pieces have insanely low level req’s compared to other gear. This is what I keep around for leveling:
Arctic Gear: Armor, Belt, Gloves: Req Lvl 2: You can use it almost immediately and it gives +10% IAS, which is essential at low levels for any character since all casters are effectively hybrid melee-casters until you get to Lvl 27 and can use Insight to replenish mana. This partial set also has a huge life bonus (+50) which is ~100% of your health at this point in the game. The health allows you to put all your stat points into strength so you can use Sigon’s pieces that come available at Lvl 6. Also has 40% MF!
Hsauru’s Iron Heel: Req Lvl 3: 20% FRW, 25% FR – It’s very hard to find any boots with 20% FRW if you’re under lvl ~25 and I hate moving slow
Sigon’s Boots, Belt, Gloves: Req Lvl 6 (though usually Str limited till a bit later): 30% IAS, 10% LS, 50% MF, 20% FRW plus resists–this is an awesome set! I use it till the end of Nightmare on melee builds; till ~Lvl 40 on casters. Sigon’s Shield is good to have on WS to give +skill to buffs until you can get Spirit.
Isenheart’s Lightbrand: Req Lvl 8: With 20% IAS and +10 dmg this is the best 1H melee weapon in the game until the rune word Steel is available
Angelic Ammy, Ring, Ring, Armor: Req Lel 12: +100% MF!!, +1 All Skills, +105 life, +12 replenish life, 50% FR, unbelievable AR boot (awesome for tanky builds with 0 dex investment). I usually run this with Sigon’s till the end of Nightmare. Add 3 PT crown for additional +72% MF, total of 222% MF with very cheap gear that gives all the other bonuses you need. And, this really shines when you play with a Tesladin or Frost Zealot and can use the Angelic saber instead of the armor. Replacing the armor with a 4 PT Gothic Plate gets another 96% MF for a total of 318% MF; with a Rhyme Paly shield (343% MF total) you have a great Nightmare farmer to get gear for that, seemingly impossible, leap to Hell.
Other stand-alone pieces not yet mentioned include:
IK’s Ogre Maul: Very good weapon, especially for Druid shape shifters
Griswolds Heart (Paly Set Armor): Is great for melee MFing as it has 3 OS and a Def ~1000. Works great with a Maul druid.
Griswolds Redemption (Paly set scepter): Has 40% IAS and up to 4 OS. With 4xShael can have 120% ON WEAPON IAS. This enables some melee builds to hit their highest BP
Natalya’s Totem: This is a good mid-level helm and especially useful for helping Merc’s meet Str and Dex req’s. It has up to 20% All Res, +20 Str and +30 Dex! Socketed with a PA for an additional +10 Str, you can actually get an Act III Merc strong enough to use an eth Spirit shield
Dangoon’s Teaching (from Ondal’s) and Sazabi’s Cobalt Redeemer: both decent 1H melee weapons. Not BiS, but they’ll work in Hell until you get something better; and Redeemer is good for an Act V Merc.