Via Who's Extreme: Bush or Kerry? and A Far, Far Lefter Place and this page, here's a listing of the lifetime Americans for Democratic Action ratings of various senators, in order from least left to most left:

1 - Phil Gramm
5 - Trent Lott
69 - Bob Graham
75 - George McGovern
76 - Joe Lieberman
78 - Hubert Humphrey
83 - Dick Gephardt
86 - Dianne Feinstein
88 - John Edwards
88 - Carol Mosley-Braun
88 - Ted Kennedy
90 - Walter Mondale
90 - Dennis Kucinich
92 - John Kerry

But, don't worry, there are still at least two others further left:

96 - Barbara Boxer
99 - Paul Wellstone