As a sample, a non exhaustive list of apps on my phone:
- ebay: useful for bidding/keeping track on watched items, keeping track of items currently selling
- midomi: useful for finding out what you're hearing on the radio, or the tune stuck in your head (I also have shazam)
- ifooty: useful for following minute by minute reports of matches when forced to be out during play
- fStream: radio over the internet - all the BBC channels for example (or any other radio station that also broadcasts via the web)
- tweetie: twitter (say no more!)
- Last.fm: as per desktop
- wordpress: should use this more than I do to post to blogs
- wikipedia: again, could use Safari and often do, but a direct link to the wikipedia infobase
- flickster: look up movies, get trailers, reviews etc
- rMote: control XBMC from iPhone - useful for dedicated media pcs
- uStream: never use this
- Evernote: photograph stuff like recipes, cool signs, nice design - tag it and bag for later viewing in desktop app or over web
- WhatTheFont: same as website, but with ability to take photo and upload directly in the app - instant font gratification
- Prowl: only useful if you use a mac - this tells me when FTP has finished uploading, when I've got a new mention on twitter, or when Firefox has finished downloading files etc etc etc
- Remote: useful for controlling iTunes when out the office but near the remote speakers

- Pocket Guitar & Guitarist - Jee persuaded me on these - I don't honestly use them very much, but they're quite cool
- TED: if you know the site, you understand why
- pic2shop: photograph barcodes to get details of item later...
- RSS Flash G: RSS reader that syncs with google reader - very handy.
- Various games including Columns (don't work no more since updated version installed though

), MOnkey Ball, Crash Kart etc etc
- google: this is of limited use to me as I don't use gmail
- Various news apps: sky news, NYTimes etc
- Various eReaders: Stanza, eReader et al
- iTalk: voice memo - although Apple have since included a voice memo app within the standard build so I stopped using this
And finally, I've got a couple of apps I'm testing (in development), but I can't talk about those

I could live without quite a few of the apps, but there are some (like tweetie, Midomi, Remote, Prowl and eBay) that I use several times a day and would miss.