There are many modern examples of IUIs, the most famous (or infamous) being the Microsoft Office Assistant, whose most recognizable agentive representation was called "Clippy".
Generally, an IUI involves the computer-side having sophisticated knowledge of the domain and/or a model of the user.
Probably the earliest examples of what could be considered true IUIs appeared in the Intelligent Computer Assisted Instruction (ICAI, aka.
Later IUIs, such as Clippy, are more statistically-based, using machine learning methods to decide how to tune the interactive experience to the individual user.
Research in intelligent user interfaces is published in general Human-Computer Interaction conferences and journals such as CHI or UIST as well as in some artificial intelligence research channels such as those hosted by the AAAI, but most importantly there are the dedicated conference series on Intelligent User Interfaces (since 1988) and Recommender Systems (since 2007), as well as the journal User Modeling and User-Adapted Interaction (since 1990).