Two places that I know of so far are:
Suse Studio is in alpha stages right now. Its quite comprehensive allowing you to chose from text only, minimal x server, and two other base templates to build upon. From there add the packages you want from category or search group. Even the desktop environment. Suse Studio allows one to build the image on their server, as well as test the image through your own web browser (very impressive! they even put browser buttons up for F1 - F12 keys). This is done though a vncview of some kind. After you done building it you get choices as to what format you wnat to download your image in.
Suse Studio is the most comprehensive build service I have seen. Watch the demo movie on their site and sign up for the alpha program. I have built a 220 megabyte minimal x image with a lightweight desktop but I screwed up and grabbed the wrong kernel. I have to wait a period of time before building another so pick and choose carefully if you do get that far.