The vos form that elfbabe describes is actually not only found in Argentina, but also in the neighboring countries of Paraguay, Uruguay, and in Central America with the exception of Panama. It also is sometimes used colloquially in various parts of Latin America. It is very surprising to me that this form is usually not taught in Spanish courses. I have taken Spanish for eight years, and didn't hear about vos until about a year ago. While I can recognize and comprehend the form, I would not be able to reproduce it. Note the similarity with the Portuguese pronoun você.

The regions that use the vos form, as far as I can divine from my readings, appears to only replace the nominative/subject-form: te remains the object form, tu the posessive.

Some sample conjugations using vos:

quedar to stay
vos te quedás

tener to have
vos tenés

ser to be
vos sos

saber to know
vos sabés

You will notice that stem-changing verbs do not appear to undergo a stem-change in the vos form. Also notice that they are generally accented on the final syllable.