A few I could name off the top of my head...
-Hemichromis fasciatus/frempongi/elongatus (fasci's get a little bit bigger, maybe 8-9 inches. hyper-aggressive little fish, they hit fast and hard. best kept with other similarly agile monsters, like the 3 centrals listed below)
-"Cichlasoma" salvini (usually 6-7 inches for males, I had a verified 9 inch male, seen much larger ones as well. highly aggressive, can hang with the likes of grammodes/istlanum)
-"Cichlasoma" istlanum (probably the nastiest thing central american that you can get under 10 inches... the "mini-umbee" nickname says it all)
-"Cichlasoma" grammodes (another tough customer with dovii-like looks to boot. not as outwardly aggressive as istlanum, but can easily hold its own)
-Neolamprologus christyii (this little tang cichlid is probably the nastiest thing on the small side of 10 inches. by far the most aggressive fish i've ever kept- breeding pairs take aggression to a new level - spawning festae pale in comparison)
-Neetroplus neematopus (underrated little fish, very interesting to watch, will make short work of most tankmates when spawning so be warned)
-Telmatochromis dhonti (the only other fish I had that came close to matching N. christyii aggression-wise. stays smaller though - still, keep alone or in a large tank with the assumption that all tankmates are disposable. when I kept these little tanganyikans, I used neets for target fish

)
-Cryptoheros nigrofasciatus (good ole convict cichlids. not the nastiest thing out there, but definately have enough spunk to lighten up a medium sized tank of similarly robust fish)
-Herichthys bartoni (nasty little guys, especially when breeding. the "tuxedo" look of breeding specimens is downright gorgeous)
-Amphilophus zaliosus (midas-level aggression in a much smaller fish- needless to say they are very tough customers)