What are all the friendships found in Julius Caesar, and are the friendships beneficial for one party or both parties?

Interesting question--it forces the reader to consider what a real friendship is. Cassius and Brutus have a relationship that is friendly enough to include trust.  As the play opens, Cassius trust Brutus enough to confide in him what he is plotting and why he wants Brutus to join in.  In regards to their relationship, it seems that neither man ultimately benefits from the relationship.  Both men lose everything and everyone and do not even accomplish what they set out to do.

Antony and Caesar have a relationship that would appear to be a friendship to most.  However, Antony has everything to gain from his relationship with Caesar, and Caesar almost seems to play on that loyalty without truly considering Antony a friend--he treats him more like an errand boy. Antony certainly benefits from the relationship that he he established with Caesar, but again, he put himself in that position and used his speaking and leadership skills to take advantage of the turmoil created by Caesar's assassination.