Documents To Go ($14.99)
Documents To Go by DataViz allows you to view and edit Word, Excel, Powerpoint, and PDF files. It is huge for opening email attachements or making notes and it edits excel sheets with ease.
It is the only paid app on this list at $14.99 and it is well worth it.
Dropbox is the best way to share files between all of your computers and devices, mac, PC, Android, iPad, iPhone, whatever. You get up to 2 GB free which is best used for files not music. You can upgrade to 50 GB for $9.99/month.
The files do not automatically download on your device so don't worry about losing space on your Android phone.
Springpad is what I use for keeping track of all things I don't want to forget, you can use images, scan barcodes, etc. You can read my opinion on Springpad here.
Google Voice (Free)
Google Voice is now open to everyone! You too can now enjoy the best way to do phones, ever. Ring all your phones with one number, voicemail transcribed to text, voicemail sent to your email. Cheap calls overseas and easy and free conference calls.
This one is for fun. As a sales rep you are probably on the road a lot just like I am. Foursquare is a nice way to keep track of where you've been and eventually more places (like Starbucks) will give discounts to people who have checked in, done "tips", or are the mayor of their location.