FontAgent Pro is my current font manager of choice. It was the first to come out with plug-ins for Adobe Creative Suite CS5. They bill it as "The World's Smartest Font Manager". Maybe so. I haven't had any problems with it. By Insider Software. $99.95. For Mac and Windows
Suitcase might be the grandaddy of font managers. Suitcase Fusion is merger of SuitCase and another program I used to have, I think it was called Font Fusion. Creative Pro has a review up here. Maybe v3 is better, but I didn't care for v2. By Extensis. $99.95, For Mac and Windows.
For a while, FontExplorer was a free product, and it was great. They eventually came out with a paid version, which I haven't tried. Macworld has a full review here. By Linotype. $79. Mac only.
FontCase is the new kid on the block. It looks pretty sweet, and the price is almost half of other font managers. By Bohemian Coding. $53. Mac only.