Sierra Club is picking up all kinds of flack for endorsing Republican Senator Lincoln Chafee. Chafee's had a few bad votes, like every other politician, but overall has been a good environmentalist.
Firedoglake, Kos, and Atrios have all been after Sierra Club, but to the extent they criticize SC for supporting someone who voted in Bill Frist as Senate Majority Leader, they're attacking it for not being a wholly-owned subsidiary of the Democratic party, which is a ridiculous basis for the attack. (Might also be worth remembering that Frist's Republican competitors for his job were even worse than him.)
Two other points worth mentioning: while Sierra Club is non-partisan, it's no secret that its membership and likely its leadership skews somewhat to the left. When SC endorses a Republican, they're overcoming a bias, so the accusations flung against them ring hollow. Second, if you read this comment thread, it's pretty clear that the people who actually know something about Chafee are much more sympathetic to the endorsement than the fire-breathers. More evidence suggesting the fire-breathers and the bloggers got it wrong.