Picasa for mac was released the other day! Click here
Picasa is a photo viewer, manager and editor built into one - And its free! If you work with lots of photographs this is a great way to organize and view them all.
The editing feature of this program is rather limited but it will do basic things like basic color alterations, crops etc.. This software cant really be compared to adobe lightroom and apple aperture as these are seen as "pro" software solutions and although they have more features they cost a lot more also.
Compared to iPhoto I think picasa is much better. the main thing I don't like with iphoto (and aperture) is that apple locks your images into its mac proprietary apple library so it cant be used on non osx operating systems.