John Chow's blog had a guest post on 10 apps for switchers for ex-Windows users with a new Mac. Having just helped someone do exactly this, I thought I'd compare notes:

QuickSilver
I'm not a QS fan. It's a wonderful program that shouldn't need to exist. It achieves much to compensate for missing functionality and productivity tricks in vanilla MacOS and that's the point, I think the basics should be in the base OS. Thankfully, after 2 releases where it was between poor and useless, Spotlight now works acceptably well in Leopard and is more than good enough as an Application launcher. QS is better but I don't want to run a separate application and manage updates for such core functionality.

Ecto
I tried it and hated it. It didn't integrate with Blogger's draft mode and ended up messing up my blog. I still write drafts as text, usually in TextWrangler which is open most of the time as a general purpose text editor. Still, one to watch for the blog author as such issues get sorted out.

Adium
Yes, fully agree. Use the OTR package for secure messaging as well.

Skype
Yes. Personally, I ditched iChat as soon as Skype supported the iSight. It just works.

Wallsaver
Ugh. Each to their own I suppose. I'll let people find their own eye candy.

ImageWell
I've seen this recommended and It does seem to do what it claims, but my image editing always requires something more functional. I've been using GraphicConverter for years, exclusively since I lost PhotoShop v4 (Classic only) in the move to Leopard. Pixelmator attracted a lot of attention with a handsome interface, but it lacks functionality. One to watch, maybe.

CyberDuck
Yes, and an honourable mention to Fugu as a capable SFTP client as well.

SnapzProX
Don't need it.

Keynote
I bought a family pack of iWork which is terrific value compared to Office and
installed it everywhere but I still seem to keep needing to send out PowerPoint. Oh well.

Spaces
It is good, but it suffers from feature anxiety. It can be setup with many screens which is potentially confusing, yet simple features like a contextual menu off a window's frame to move between Spaces and most usefully Show In All Spaces is a notable ommision. Definitely version 1.0. The jumping around as it follows updates windows can be nightmarish for power users moving quickly.

I also recommend:

GMail with IMAP for e-mail.

MacFreePOPs to bring webmail into your Mail.app inbox. Use the preferences to close the foreground application after launch leaving the freepops daemon running in the background.

HimmelBar for application launching, but Spotlight is taking over quickly for me. I used to use TigerLaunch as well but I dropped it with Leopard.

Firefox. Safari keeps getting better but it's always good to have a few browsers around and if you are doing any web development, this is the only choice.

VLC and mPlayer for all your video playback needs. Some people like Perian, so I'll mention that also.

RSS feed reading can be done in Mail.app for most people.

For switchers, I'd recommend using DashBoard, but personally I have it disabled and only use Yahoo Widgets (nee Konfabulator). Apple didn't copy enough of Konfabulator when they implemented Dashboard; I want widgets at the desktop level, permantently visible not in a separate, virtual plane only reachable by an F-key which then hides my open windows.

And last but not least, use Secrets to get at those hidden system preferences, much easier that using Terminal and default write ...