Traditional responsibilities of a bestman include:

Organizes any prewedding party for the groom
Coordinates the ushers' gift to the groom and usually gives individual gift to the couple
Attends the rehearsal and rehearsal dinner
Gets the groom dressed and to the ceremony on time
Makes sure the groom has the marriage license with him at the ceremony
Delivers any payment to officiant, sexton, and ceremony musicians, as prearranged
Enters the sanctuary with the groom
Holds the bride's wedding ring during the ceremony until the vows are spoken
Makes sure all ushers are correctly dressed and in place on time
Walks in recessional
Witnesses the signing of the marriage certificate
Welcomes guests at the reception
Offers first toast to bride and groom at the reception
Dances with the bride, maid of honor, mothers, and guests
Helps the groom get ready for the honeymoon
Gathers up and takes care of the groom's wedding clothes after he changes
Has a car ready for the bride and groom to leave the reception, or perhaps drives them to their next destination
Pays for own wedding attire and transportation to the wedding