I have very strong opinions about covers. Bad covers are musical crimes. Ugh. Let's see...
Creep (Radiohead) by G4
I think we're alone now (Tiffany) by Girls Aloud
Iris (Goo Goo Dolls) by Ronan Keating
Proper Education (Pink Floyd) by Eric Prydz (yes, it's technically a sample, but it's still crap)
American Pie (Don McLean) by Madonna
As a sidenote, songs by the following bands should not be covered unless you are as good as or better than the bands in question. Chances are you are not. You will not improve on what the bands have done themselves, so just don't touch. The list:
The Beatles
Abba (though I did hear a moderately entertaining cover of Does Your Mother Know by Ash)
Queen
Radiohead
U2 (exception - Johnny Cash's version of One)
Green Day
A possible all encompassing exception could be made for Weird Al Yankovic to cover just about anything, but he'd have to run it by me first just to make sure.
Just to show that there are some covers I like...
Common People (Pulp) by Ben Folds and William Shatner
What a wonderful world (Louis Armstrong) by Joey Ramone
One (U2) by Johnny Cash, as mentioned above
Tainted Love (whoever) by Marilyn Manson
Feeling Good (Nina Simone, I think) by Muse