Google Gmail shortcut keys

In Settings, enable keyboard shortcuts.

j (move to older thread; aka. Conversation)
k (move to newer thread)
o (open thread)
With thread open:
n (next email in thread)
p (previous email)
o (open/expand email (or collapse it if it currently expanded))
u (return to list view)

y (if reading Inbox, archive thread; ie. remove from Inbox)
s (star or un-star message/thread)
# (delete; ie, send to Trash)

x (select thread -- useful in thread list view, in conjunction with j & k shortcuts)
? (Help; ie. show list of keyboard shortcuts)

Link to full list of shortcuts:
http://mail.google.com/support/bin/answer.py?hl=en&ctx=mail&answer=6594

Thanks to BT for the reminder!