I am a big fan of simplification, and most of my apps lean to the work side of things. Here are the ones I could not make it through the day without. There are a few paid apps on this list, but I consider them worth it, and if you get Amazon App Store, you can score some good ones free.
Evernote. You have no idea how much you need this until you have it.
Dropbox. Sharing a work document or family photos, this app saves more time than any other I have found.
Hootsuite. Social Networking at its easiest. Faster than Seesmic, with easier interface.
Business Calendar. Best interface, best widget, easy to use. No downsides here
SwiftKey. BEST keyboard app ever. Learns your word patterns and predicts the next likely word for you. After about a week, texts write themselves.
Checkbook. Great money managing app, unlimited accounts on the paid version, and backup/restore data options.
Slidecreen. This is one you either love or hate. If function is more important that fashion to you, then try the free version and see.. It gives you feeds for your most important info on one screen. FB, Twitter, text, email, calendar events, stocks ect. but it is a purely functional app. I see this app coming into its own within a year, as they are working on APIs that will allow any app developer to interface with it. Once that happens, it will take off like a rocket.
Autokiller Memory Optimizer. I am anti Task Manager, they kill your battery and most don't work. What I like about AMO is that it is not really a TM. It does manage some processes, but for the most part it flexes your memory based on demand. Great app for ending lagging on your phone. One note, this is for Rooted users ONLY.
Now my two guilty pleasures:
Amazon Kindle. I really should not have to explain this, just be careful when reading in the bathroom. It's not like dropping a book in the sink.
Redbox. I used to be a manager for Hollywood Video. After seeing how much Redbox has improved over the last 2 years, it is no shock that HV went bankrupt. Search movies by location, reserve right from your phone, this is too damn easy for 1 dollar a day rentals.
Feel free to add to this discussion with your favorites.