ABC Banners - freely accessible banner maker once again. Check them out.
Crecon.com
QuickBanner - offers paid to free banners and buttons. Look carefully at which product you want to make.
The Banner Generator
The Logo Shop - presented as a "competition", The Logo Shop appears to be a one-man operation doing customized work. So the banners take awhile to create and be sent to you. If that's okay with you, try out this service.

Online Logo Creators

Although the following are not really banner-creations sites, you can create close-to-banner size logos here as well as your page header graphics:

FlamingText
CoolText
MediaBuilder 3D TextMaker - not your typical banner style. Probably best suited for headers rather than banners.

Also check out the Flash resources page. Some of those sources have free "Welcome" headers created with Flash. And don't forget the Xara3D Header Maker on this site.