Omnidirectional antennas

I'm looking for suggestions for an omnidirectional FM antenna that I can mount on my roof. I live in an urban area that is relatively free of hills and tall buildings.

I'm using a Fanfare FM2G-C and it works great:

Marv Southcott was very helpful and a pleasure to work with.
In addition to the Fanfare suggested by Armstrod, you could look at the Magnum Dynalab ST-2FM. They're pretty much identical. I would also suggest you look at Terk antennae as they are antenna specialists.

If you live in an urban area which is not prone to multipath interference, and with no major selectivity or sensitivity issues regarding the stations you listen to, you may not need an outdoor antenna at all. Terk also make some nice little indoor units that may do the job for you. I use one called the AM/FM Q. It's amplified, tunable, way cool to look at (although beauty is in the eye of the beholder of course) and I didn't have to run a wire for an outdoor whip antenna. Cheaper too.
Here's an interesting note about the FM2G and the ST-2:

I also agree with Markphd that a Terk might be all you need if you're in a strong signal area and you can position the Terk near an outside wall. However, my experience is that the passive Terks work better than the amplified ones. I found the amplified Terks to overload my tuner's front end on strong signals, and added a lot of noise.

Where I live now in the Hudson Valley, my Terk was useless. The FM2G-C worked wonders, even though I can only bolt it to the windowsill of my 3rd floor apartment. Roof mounting should be ideal.
I am using the Fanfare FM-2G FM antenna inside and it works great on most FM stations on my Magnum Dynalab MD-102T FM Tuner. My antenna is located on the right hand side and behind my stereo rack. My 54" Fanfare is bolted to an large steel bracket so that it stands straight. Reception is excellent on most stations but some do not come in okay. Marv Southcott from Fanfare was very helpful during the purchasing process. Please see and their FM Antenna section for more details.