Reservation system – Use cck_validation, date, calendar, & workflow_ng to create a system that allows for reserving rooms or other resources. The above link has php code for checking for time conflicts. Use the very latest 5.x dev releases of calendar & date.
Group wikis – Modifications to the freelinking & wikitools modules so that [[Bracketed Links]] go to wiki pages in your own group, rather than showing all wiki pages by that name in all groups. Use the very latest version of organic groups (og).