Wernher van Braun designed a complete manned Mars mission in 1969 landing a crew of six on Mars. The study was based on previous NASA studies with Boeing at much lower cost

Dr. Robert Zubrin, Mars Society president, is fighting since the early 90′s for his project of a “Mars Direct” manned mission, using mostly existing space technology and avoiding expensive and time consuming assembly of hardware elements in low earth orbit. Here are is main proposals for such missions between 1991 and 2011:

Zubrin with SpaceX to Mars_May 2011

ATHENA Mission_low cost to Mars_Zubrin_1996

Near Term Mars Mission Options_Zubrin_1993

Mars Direct Scenario_Zubrin_1991[1]

A overview on Mars exploration studies from JPL including document download capability can be found here