Making an appointment within the enterprise is not so hard these days. Everybody has their Outlook/Exchange or Notes/Domino system in place, nicely connected to their blackberries. In the more informal space, it is much harder to get this all done. Sure I run google calendar synchronizing with my blackberry using google sync and anyone can invite me, but the process of actually agreeing on the appointments date and time with a group of  people not in one enterprise is a serious PIA. Think of me with my clients, or the meeting of the local neighborhood council, or anything. Here is a little internet gem called datumprikker.nl (Dutch for date-poker). Enter your proposed dates, enter the contacts you want to have in that meeting, and let the system do its thing. It is simple and effective and does not try to use ones electronic diaries. It does provide an iCal file when everything is said and done. Brilliant through its simplicity.