Wilson Rothman has a very solid list in his post "The first Windows Phone 7 apps you should grab though I would add a few to this

• Seesmic – I know Twitter is on the list above but Seesmic has some cool features
• TED – you gotta have inspirational talks with you
• Marketplace Search – handy for finding stuff in the marketplace on your phone (the Zune PC client has search)
• Microsoft Tag – just because