I tend to be a lone wolf, so my go-tos are Dropbox (mostly for presentation and teaching files), Evernote (for notes-to-self, because only just the other day did it become possible to edit .txt files in Dropbox on my iPod Touch), Zotero, and Google Docs (for the occasional oddball need).
I've had fair luck getting techno-unhappy colleagues to collaboratively edit GDocs. Doodle for scheduling is also a godsend, particularly if your project crosses institutional (calendar) lines.
I have a visually-oriented colleague who swears by mindmapping software. It's not my thing, but it might be yours.
Edited to add: The hardest thing, honestly, is meeting agendas when your meeting includes people who don't do any kind of portable computing. Also, in my unhumble opinion, people who email ordinary, non-formatting-dependent documents like minutes or agendas as Microsoft Word attachments are earning lots of bad karma thereby. Plain text is good!