(all words are synonymous, mean "cool, good, great", given in alphabethical order)
зачот - in written form used with "O" instead of "Ё" (and capital: "зачОт"). Developed in Internet blogs from the words "зачёт", "зачтено", "засчитано", etc. In its normal meaning mostly used in college&university speach.
зашибись - a form of the verbs "зашибиться", "зашибить", but actually it's not important what do these verbs mean because this word was formed as more or less decent analogue of an obscene (матерный) word. Also used in opposite sense.
здорово - just great.
зыко - out-of-date word.
классно - real class!
клёво - just nice, cool.
козырно - from "козырь" (trump card), note the place of the stress.
круто - just cool. From "крутой" - tough, hard.
кульно - from "cool".
мега - just single "mega-" like in "megamall", f.e.
офигенно, офигительно - from "офигеть"; also the decent analogue of an obscene word.
прикольно - from "прикол", something "cool, interesting"
супер - just "super"
темно - from "тема" (theme, subject). Тема is usually "дикая" (wild). (Видел новую Нокию? - Дикая тема! (Have you seen the new Nokia phone? - Yeah, very cool!)