Offline, the most used Project Management software is most likely Microsoft Project, now it's it in version "Project 2007". There are so few players when it comes to offline project management and the rest of the competitors are either too niche oriented or are not worth mentioning.
As for online Project Management software, there are so many right now (over a 100), and all of them are very similar to each other, what's different about them is just the price. If you want to go with an online PM tool, just go for a one that has good support (pay attention when reading reviews, as most of them are paid), the features you want, and daily backup.