dojo is an open-source Javascript Toolkit which is designed to facilitate integration of advanced Ajax and DHTML features into web applications. CiviCRM has started incorporating Dojo widgets as of 1.6 - including:
Auto-completion on Quick Contact Search block. Tab containers for more efficient handling of Contact Summary tabs (doesn't reload the entire page when switching between tabs). Mouseover Tooltips for inline help.... and we are planning on adding more features in 1.7 as we work on streamlining the user interface and creating more desk-top like work-flows.