PhotoImpact by Ulead.  This is our current favorite.  You can create really interesting text graphics very simpily.  All the the page headers at our site were created with PhotoImpact.

Paint Shop Pro by JASC.  This is an old favoriate.

Adobe PhotoShop.  The is an extremely high end (and expensive) graphics package.  It does a great job, but you have to be willing to take the time to learn it.

The Black Box by Alien Skin.  This is a set of PhotoShop filter plugins.  You can get really neat special effects like drop shadows etc.  These plugins will work with the above three packages.

Pixelsite is a website that lets you create great graphics and then download them for use in you web pages.  It is slow at times, but worth the wait.