Must have HTC One Apps?

[G-Man]

I can't believe no one has mention GTalk.

Isn't G-Talk a default app? (unless you're running CM) ?

[UltraDroid]

Yeah, I think it's automatically installed as part of the Google Apps.

[Ssscrudddy]

I already mentioned Avast antivirus for android. Even better now my phone is rooted. If your phone is nicked or lost, you can remotely control it over sms to do various things, eg wipe all your data, permanently lock it, set off a siren, enable gps, text you the new phone number if they change the sim card... & much more.

Team viewer - remotely control your pc with your phone
ES File explorer (by Estrongs inc) - Also connects to dropbox googledrive skydrive & any ftp server you want, as well as your LAN over wifi, zips & encypts
AdFree Android (by BigTinCan.com) - Rooted phones only (removes ads)
Swype Beta - a keyboard works great on the V. (not on the market place)
AirDroid - wirelessly manage your phone from your PC browser.
Jays Headset Control - skip tracks volume up/down etc. for 1 button headphones

[Funkadube]

There are so many I like, but the most useful for me are:
FoxFi (free hotspot app, not on marketplace for AT&T but you can d/l and install from web)
Tango and/or Skype for video calls
Plex (stream movies, tv, pictures from computer...no need for a big HD anymore)
TuneIn Radio / Slacker Radio
Netflix
ES File Explorer & Chrome browser are quite useful as well.