My favourites are
Evernote
Docs to Go
Goodreader
Expensify
Dropbox

What else is worth looking at?

Ta

If you are into guitars, get an iRig

just got garageband, that is one very sweet piece of software.

apart from the ones you list above, my most used apps are:
penultimate
djay (f’ing brilliant fun with friends and a few drinks!)
wunderradio
photobucket
teleboy TV
keynote
idisk

and games
X2 footy
space miner HD
reckless racing HD
NOVA

And various interactive books for the kids, Dr Suess ones are good (Lorax and Grinch are very good), Solar System is brilliant.

HTH!
Kev

Simplenote rocks my world.
as does iAnnotate PDF
Twitter
keynote
iplayer
byline
instapaper
BBC news

games wise:
monkey island 2 SE
broken sword

and a bunch of others, they’re the stand-outs though.

Here is a tip on how to get iMovie on to your iPad

Chopper2
Audiogalaxy
Asphalt6

flight simulator – space shuttle! wow

and here is the link I forgot earlier

http://www.macrumors.com/2011/03/10/installing-imovie-on-the-original-ipad/