Swype or Mobile Write (I'm an old graffiti guy, so it's Mobile Write for me)
NoteEverything
GDocs
A Online Radio
Barcode Scanner
Beamreader
Contact Search+
Network Monitor
ShopSavvy
Simple Weather
Wapedia
Where
But I'm using a Motorola "Droid" (a855), so I use lotsa other progs that might not apply to you. Documents to go has a lot of fans, and I liked and used it on my Palm devices, but I don't care for it at all on my Droid. Give it a real close look and compare it's use against other free stuff 1st.
I was all jazzed about my Droid for a while, but the honeymoon is over. I think HTC has better products than Motorola, so don't let all the hype make you think that your Eris is 2nd string or somethin. If you "have work to do" and hope to manage 500 contacts, lots of memos, do business related scheduling and to-dos, I'd advise really giving your Eris a workover before your 30 days is up. My old Palm Treo was a better tool for business than my Droid is. Facebook, GPS, and Youtube are fun, but don't contribute much to gettin' stuff done.
Welcome to the android crowd and the forums, and by all means have fun with that doo-dad!