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, not the same thing as the somewhat slangy English word in this sentence conveys. Some larger dictionaries include words such as guay, but that's hardly the only word that can be used.
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.
To convey the idea of "cool" in a more colloquial way, you'll need to go beyond what most Spanish-English dictionaries offer. To find out how the idea is conveyed in the language that Spanish speakers use in real life, one of our forum participants recently asked the question, "How do you say 'cool'?" What follows is a considerably edited version of that discussion (and the posts originally written in Spanish have been loosely translated to 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, está del *putas*, es una verraquera and many others; but these are Colombianisms. I'm 21, so I'm not a teenager anymore. I've heard also "qué *putería*, and 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."