The list at TUAW is pretty extensive but here are more things you can say to Siri:

Texting/e-mailing people

“Tell my brother and my girlfriend I’m late”

So you can text/mail multiple people at once, isn’t that cool?

After dictating a message:

“Change it to I’ll be right there”

Changes the message.

“Add I hope”

Adds a sentence to the message.

Calender

“What’s on my agenda one month from now?”

Shows you what’s on your calender one month from now.

Alarms

“Wake me in 20 minutes”

OK, this kind of is on TUAW’s list (“Wake me up in 8 hours”) but this one is great for taking short naps!

Reminders

“Remind me in 10 minutes to close the windows”

Creates a reminder with a due moment ten minutes from now.

“Show me all my reminders”

Shows you a list of all your reminders

“Show me my reminders for wednesday”

Not sure when I would use this.

Wolfram Alpha

“How many calories in one chocolate bar and two bananas”

I’m stunned.

Notes

“Show me all my notes”

Shows you a list of all your notes.