Edit General Slang
legal (Leh-gal) :cool
nossa (noh-ssah) :exclamation like Wow
puxa (also puxa vida) (puh-shah) :exclamation like Wow
muito louco (mooeen-to lowco) :cool (or funny)
gringo (greengo) :foreigner
gatinha (gah-teen-ya) :babe, hot girl
Cê (seh) :Diminutive for "você" (you)
Qualé? (kwal-eh) :Wassup?
Beleza? (Beh-le-zah) :Waasup? (Positive way.)
Massa (mah-ssah) :cool (Same as legal)
Grana, dim-dim, cascalho, mufunfa :money
chapado (sha-pah-doo) :drunk, stoned
Bagulho (bah-goo-lyo) :Ugly women, drug (very rude)
Perua (pee-roo-ah) :Rich Women (Negative), Sedan car
Mascar (mas-kahr) :to lie, to talk to a girl with second intentions (rude)
Nó (noh or Nooh) :Diminutive of "Nossa"
The above list of expressions can be understood all over the country, but it doesn't mean you'll hear people speaking this all around. 'Legal', for instance, is far less used in the Northeast region.
Edit Northeast region (Alagoas, Bahia, Ceará, Paraíba, Pernambuco, Rio Grande do Norte, Sergipe)
Oxe, oxente (osh - osheh-n-tee) :Why, as in "why! that's strange"
Vixe, vixe Maria (vish - vish mah-ree-ah) :Holy mother! (comes from Virgem Maria -- the Virgin Mary)
Edit Rio de Janeiro
sangue bom (sahn-ghe boh-n) :Nice man
Edit Rio Grande do Sul
tri-legal (tree leh-ghal) :cool
tri (in many contexts) :cool
Edit Sao Paulo
truta (troo-tah) :friend
Sampa (sahm-pah) :Sao Paulo
Rolé (rool-eh) :"a ride." (e.g. Vamos dar um rolé. - Let´s go for a ride.)
(Note: If you talk in slangs with the young ones, be careful because they are divided in "tribos"(meaning: groups) like skaters, surfers, etc. A slang spoken by a certain group may sound senseless to another one.)
brother, brodi, brou (same pronoun as in english) :friend, dude
cara (car-a) :men
meu (mew) :men
Edit Minas Gerais
Uai (why) :Doubt Exclamation (Used also as an emphasis.)
Trem (treh-eem) :Used as a substitute for "this" (e.g.: "Que TREM que é esse?" - What is this? )
Sô (soh) :Emphasis per se, it´s an early alitheration for SENHOR(Sir). E.g. O quê que é isso, sô? - What is this *sô* !!