This is a list of the highest single-season ERA's that can be quantified in numerical form, because quite a few pitchers have actually had worse seasons. There have been 48 times when a pitcher has allowed earned runs in a season without recording an out, which is ERA of infinity. The all-time leader in this category is Bob Kammeyer (Yankees) who gave up 8 ER in his one relief appearance in 1979, but failed to record an out.
Highest Single-Season ERA
189.00 Jeff Ridgway, 2007 Rays (.1 IP, 7 ER)
189.00 Joe Cleary, 1945 Senators (.1 IP, 7 ER) (The only appearance of his career)
162.00 Joe Hudson, 1998 Brewers (.1 IP, 6 ER)
162.00 Tom Qualters, 1953 Phillies (.1 IP, 6 ER)
The 2010 leader is Jeff Samardzija (Cubs), whose current ERA is 108.00 (.1 IP, 6 R, 4 ER), but he should be able to bring it down soon and avoid becoming a part of this list (at least he has recorded an out).