1. This may be obvious but as of the latest version, you don't gain much from a corporate email program like Entourage. I use Apple Mail, with IMAP and Google Apps for Your Domain. One account has 3.8GB of email and another has 4.1GB, according to Google. It scales fine and stays in sync across devices. Plus Address Book links to Google Contacts, and iCal links to Google Calendar.
2. I prefer MagiCal (http://www.charcoaldesign.co.uk/magical) rather than Eigenclock as it includes options to put "yyyy-mm-dd [day] hh:mm:ss" by the Spotlight icon, with [day] clickable for the calendar.
3. I like SpeakTime (http://www.mecanisme.net/software/speaktime/) for putting a row of glance-able analog clocks at the lower left of my screen (semi-transparent so they look like part of the background) to be aware of alternate time zones.
4. I use Skitch (http://www.skitch.com) for screengrabs with annotations to share with colleagues/clients.
5. I use Serverskine (http://www.serverskine.com/) to keep track of server logins.
6. I use Foxmarks (http://www.foxmarks.com/) to keep bookmarks in sync across all browsers and computers.
I don't use QuickSilver, Spotlight gets the job done.
I'd also recommend the following paid apps depending on your needs:
1. iWork '09 (Pages, Numbers, Keynote) for all office apps needs with MS Office 03-07 compatibility. It's worth the $79 for the polish and usability. The "free" alternative will cost more in the long run.
2. OmniGraffle Pro for diagramming with Visio (even Visio binary file) compatibility.
3. OmniPlan for MS Project compatibility.
4. Coda for text based web development. (Yes, TextMate is in my dock. But so is Coda.)
5. 1Password for credentials management across browsers and computers
6. Things for "GTD" methodology (or OmniFocus for more features)