sadly, Keepass is not very Mac/Linux friendly, because it's built on .NET, so it's only an option for pure Windows users.
I use Keepass daily on Windows and Ubuntu and used it on OS X in the past without any issues. I highly recommend it.
I don't know about Linux, but I've been using KeePassX for Mac for about a year now. It only supports the older KeePass file format, and it's ugly, but it works great. There are also a few iOS apps you can use, my favorite of which is KyPass.