My Top 11 (because I couldn't choose just 10), no particular order:
AP Mobile - when it works...update was supposed to be released weeks ago to address crashing and has not been. But when it doesn't crash, it's the best news app on WP7)
The Weather Channel - LiveTile works better than Weatherbugs, like the app better as well though it's a bit laggy and buggy at times
Facebook - for the stuff you don't get built in. I expect to use this less when mango comes out.
The Onion 7 - Nice Onion reader, has some issues and some things require cliking through to their site to see properly, but the big bonus for me is in the Video and Radio playback, which is much faster through the app than when using their mobile site
Telenav GPS - Pay 10 bucks a month for this, can't wait for Mango and Bing turn-by-turn. I know there are market place apps I could buy for less than 10 dollars to replace Telenav until Mango comes out, but I don't trust them. Maybe I should try one out if they have demo mode.
Amazon Kindle - Nice ereader for the phone. I read public domain classics and the occaisonal splurge on a paid ebook, the prices for which are highway robbery.
Bank Of America - No brainer if you bank with them, bill pay in particular is easier in the app than on the real website. I use safepass for logon which makes logging on to this somewhat akward, Mango and Multitasking should help with that. One thing the app is missing is picture deposit. Hopefully with Mango and the camera API addition this feature will be introduced.
Windows Phone News 7 - There are actually two apps named this and I have both. They each crawl various news sources for WP7 related content and present it up. Can't get enough WP7 news so i use both.
Walgreens - Picture/Barcode Perscription Refills
Fandango - Movie Reviews / Buy Tickets
OpenTable - The Tonight for 2 slice is awesome, provides a quick way to get a glance at what is open that night nearby for you and your SO.
Need for Speed Undercover
Rise of Glory
Chicks n' Vixens
Word Tangle Free