The apps I use the most, mainly for work, are:
1Password - probably the single most useful utility I have, genuinely couldn't live without it now
Dropbox - cloud storage
Google Drive - cloud storage
Growl - notifications, slightly less relevant in Mountain Lion due to it including Notification Centre
iWork Suite - Keynote is very good for presentations, Pages is okay but a bit simplistic, Numbers is a bit ropey and rather slow compared to Excel
OmniFocus - outstanding GTD application, I basically live in here
OmniGraffle - excellent diagramming tool, way ahead of Visio
OmniOutliner - fairly decent note-taking / drafting tool, could do with better organisation
OmniPlan - decent project planning tool, but a bit faffy compared to the other Omni apps
Reeder - use this all the time for RSS via Google Reader, superb
Remote Desktop Connection - not very good, but it does usually work when RDP'ing into Windows machines
Skype - still pretty de facto in the business world despite its descent into mediocrity and ads
Sparrow - really good email client, particularly for Google Apps / GMail
Sublime Text 2 - trounces all other Mac text editors by a country mile, and I've used them all
The Unarchiver - jolly useful unzipping / un-raring / un-everythinging tool, seamless
Transmit - a very good FTP client with some cool features (folders for connections, S3 support, syncing etc.)
VMWare Fusion - still find this better than Parallels, though that might be because I use a lot of other VMWare stuff and do Linux virtualisation as well as Windows
XLD - roughly equivalent to EAC on Windows, a really good lossless-oriented audio CD ripper
I have a few others but they are not especially good 'finds', as it were