The Long Press on the G1

Pressing and Holding on:

Home Key will bring up Recent Applications.
End Key will bring up the Silent/Power Off menu.
Green Send Key will activate voice dialing.
A message will bring up a menu with Call #, Forward, View Message Details, Delete Message.
An image in the browser to save it or view image alone (also works by having the image highlighted and long pressing the trackball).
A link to open a menu with Open, Open in New Window, Bookmark Link, Save Link, Share Link, Copy Link Url (works with trackball as well).
the desktop will allow you to add a shortcut for an Application, a folder, a bookmarker, contact, Gmail Label, Music Playlist, add a Widget (Clock, Picture Frame, Search), and modify your background. If you add AnyCut you will have even more options.
A number in the call log for a menu to Call, Edit before calling, Send SMS, Add to Contacts, or remove from call log.
Menu key while powering on will put the phone into Safe Mode.

the menu key with a menu displayed will show the shortcut key for the menu options.