That is largely up to your personal preferences. If you familiar with the Java IDEs, you will notice that they run on all platforms. Eclipse is probably the most popular, Netbeans is good as well and has integrated support for maven whereas Eclipse is still a plugin. If you're using Eclipse, you can take advantage of JBoss Tools / plugins for Eclipse. That pretty much covers the good free IDEs out there.
As Walter said, Java IDEs runs on MacOSX but AFAIK best SEAM support is from JBoss Tools in eclipse.