This is a cool Spanish lesson.
How would you translate the above sentence to Spanish? Look up the word "cool" in a Spanish-English dictionary, and chances are the first word you'll find is fresco - but that word is used to refer to something that isn't quite cold. Some larger dictionaries include words such as guay as a slangy term, but that's hardly the only word that can be used.
Bueno Can Be Good
If for some reason you need to convey the idea of "cool" and have a limited vocabulary, you can always use a word you probably already know, bueno, which means "good." It's not a particularly cool word and doesn't come across as colloquial, but it will get most of your idea across. And of course, you can always use the superlative form, buenísimo, for something that's especially good.
'Cool' Words Vary by Region
There may no good Spanish equivalent of "cool" that works everywhere, but native Spanish speakers in a forum sponsored by this site offered their perspective on what may be best. Here's a part of their conversation, held originally in Spanish and English:
Chabela: How do you says "cool," like "that's cool!" What do the teenagers say? I know it can't be translated directly, but…
Cyberdiva: One word to use is chévere.
Duras: It can't be translated directly, because each country has its own versions.
VictorIm: Chévere is kind of old-fashioned (1960s). Is there anything new?
Bandini: Duras is correct. Every country has its own vocabulary for words like this. The particular word you mentioned (chévere) originated in Venezuela but due to Venezuela's major export (Spanish soap operas), the word is now become popular in a dozen other Spanish speaking countries, including Mexico.
Rocer: In Mexico we understand the word chévere, but we don't use it. Only if we talk to Venezuelans or Colombians, I guess.
Adri: When I was studying in Spain last semester, I learned from a native-speaking friend of mine that they say guay or qué guay.
Guero: I think chido and buena onda would work well for "cool."
VictorIm: Buena onda sounds old-fashioned to me. Anything with onda sounds old. Are there any new expressions?
Dulces: I have heard está chido and está padre in Mexico.
SagittaDei: A very common translation is genial, está genial. Is very widely used in the Spanish-speaking world.
As has been pointed out, there are many words depending on the country. I use está bacano/a, está una chimba, es una verraquera and many others; but these are Colombianisms. We also use the anglicism cool as in "es muy cool." "Rich" teenagers like to use English in this way. It also depends on the social level.
By the way, "eso es chévere" is less expressive than "eso es genial," the former is like "that's nice". Note that you can use either estar or ser with the obvious difference of permanent and transitional attributes.
Tottefins: In Mexico they say padre or chido on the streets. However, on Mexican television they say genial.
Maletadesueños: Here in Texas you often hear qué chido, está chido, qué padre, etc. Other people who aren't from here that I have spoken with, such as my friend who lives in Venezuela, think these expressions seem comical as they're "Mexicanisms."
Rupdaddy: I have heard the word bárbaro. Most of my studies have been of the Spanish of the Río de la Plata, Argentina. I know that in Uruguay, at least among the youth, they say de más.
Chabela: I know that in Uruguay sometimes the youth say "de más." Those words are the same, more or less, with what the youth say in the U.S.
In Mexico, particularly Tijuana, the word curada is widely used as meaning "cool." Sometimes recurada is heard. I've also heard the term chulado by people who come from Mexico City.
OjitosLindos: I think in Spain the verb molar is used like gustar to mean something similar to "cool," for example: "Me mola el cine" would mean "I like the cinema" or "the cinema is cool." I think this is only used among young people (teenagers).
Anderwm: Yes, you are right. Molar is a teenager thing. In Costa Rica and Nicaragua the people use tuane.