Since no one has mentioned it yet, I'll go ahead and plug InstallAnywhere. We use it at work for all our products and I can't say we've really had any problems with it (that weren't caused by mistakes in our own scripts at least!). It's a cross-platform GUI installer which is a nice feature if you expect to have different desktop environments in play, and it also supports console installers, silent installs, etc.
I googled around a bit and here's a review of InstallAnywhere you might find helpful. It's from a first time user, rather than a publication or thinly veiled press release, which probably makes it less biased and a better indicator of what your experience using it might be like.
