As pointed out Shadowknight is melee and debuffs but is also heavy armour with a slew of hate grabbers and therefore a Tank above all in many players minds.
However there are other not quite so obvious options for nice melee plus debuffs and here they are for ya :
1. Scout> Bard> Dirge
Dirges have sweet DPS like all the Scout classes and a LOT of debuffs and other songs that generally seriously lower the enemy strength. As you would expect a songster gets group buffs as well but this particular sub is way heavy on the debuff side whereas the Trobadour is the opposite with more buffs less debuffs.
2. Scout> Rogue> Brigand OR Swashbuckler
Ok. Brigand is Freeport and Swash is Qeynos but both have a similar sort of line. This is a high DPS melee with a mix of stuns, strikes that raise or lower hate with a mob and other Crowd control type stuff as well as debuffs lowering speed, strength, parry, agility and the like. The highlight of Brigand are the snares and its possible to snare whole groups of mobs, swash on the other hand leans a bit more towards assisting the group with movement and stuff. Coming from the rogue background this pair of subs has a wicked line on stealth too.
3. Fighter> Brawler > Bruiser or Monk
These two go in for unarmed styles, the Freeport Bruiser is a slugger, the Qeynos monk has more finesse but both are capable of seriously high DPS. They are NOT really debuff oriented as such but there are lines of combat arts in both that are debuffs but nowhere near as much as the two Scout subs above. And also, being a figter sub means Tanking and Off Tanking skills. The Monk gets some minor heals and group member wards too and the Bruiser gets more in the way of group combat buffs.
I hope this helps. Oh and dont forget that Tanking in EQ games is nothing like in FFXI. In FFXI there is ONE Provoke which wanes in effectiveness on a long recast and requires a lot of group co-ordination and assists to get it to stick, y'know SATA and all that and making sure nobody annoys the mob too much. In EQ tanking is as much an artform as method and the tank classes get a load of different hate grabbers. So, Yeah Shadowknight is a tank but its a heavy debuffer with lifetaps, a pet spell and lots of heavy armour and is well worth a look. Players who get into SK find other classes really boring, its kind of a cult thing in some ways.