I was wondering how other site designers approached the problem of running multiple subsystems (applications) on their sites. Let's say I want a site to contain a forum, a store, maybe a wiki, and other stuff. The key is that each of these applications has an ID and password (to make postings or purchases, as a non-guest). They also have their own "look and feel" to them.

Is it better to keep visitors "in the application" until they're done with whatever they're doing, or is it OK to let them jump in and out of the application (visit other applications)? Say they're in the store, and have stuff in their shopping cart -- if they go off to visit the forum (perhaps to read up on user experiences with one of the products), there's a chance that they'll forget to get back to the store and complete their purchase! So, should I discourage visitors from leaving the application until they've completed their task, or should I add links to other parts of the site to encourage jumping around? If the former, is it a good idea to give the different applications a distinctly different look and feel, to help the visitor see if they've left one application for another, or do you try to make everything look as much the same as you can? For "different", of course I'd use the same background color or image, and the same site logo, but otherwise leave the look and feel pretty much "stock" for the application.

A related matter is that each application has its own ID and password, so is there any technical problem with having a visitor signed on to multiple applications at once? I wouldn't think that the databases or cookies would "collide" in any way, but I thought I'd check for any negative experiences in this area. It would also be nice to have a visitor sign on only once, and have that signon propagate to the other applications, so the visitor doesn't have to sign on again and again during a visit. Are there any "portals" that help with this? The ones I've seen seem to be written for very specific sets of applications, and are not generally applicable. I suppose I can kludge some sort of "single site signon" to fill in ID and password for any application after signing on to the first one (or at the entry portal), and log off from all applications when logging off from one, but that sounds like a lot of work that I'd like to avoid.

Anyway, I'm open to suggestions and reports of experiences with managing multiple applications on one site. The easiest solution would be to just leave the different applications as independent systems (sign on separately) and encourage users to complete their business in one before going on to another. Does this sound like a good thing from a visitor's point of view? Thanks!