After a clean installation of the OS (or an Archive and Install, in my case) you're left with two things: a zippy computer not confounded by a bunch of third-party preference panes and menu bar items and a lot of decisions.
Even though it's great to run your operating system naked, there's also a reason you installed (most of) those silly little applications and utilities in the first place–they're incredibly useful.
That said, here's a short list of the applications that I had to re-install after a nuke and pave installation of Leopard–because I couldn't live without them:
1. Quicksilver–Say what you will about the improvements to Spotlight, they pale in comparison to QS. This application launcher slash do everything program was the first thing I installed after Leopard, after about eight minutes. Version b32 (3813) works great with 10.5.
2. MenuCalendarClock–Once I got used to having the date and a convenient drop-down calendar in the menu bar I had a tough time going back. The registered version (US$20) even displays events from you iCal calendar. Make sure that you have version 3.0.1.
3. WeatherPop Advanced–I spend entirely too much time in front of my computer so I like to know what it's like outside. Plus, it's getting cold up here in the northeast and I need to know when to bring my tomatoes in. For US$8 you can get the Advanced version which includes a Doppler radar view in the drop down. Version 2.6.1 plays nice with Leopard.
4. Graphic Converter–Sometimes Photoshop is overkill. I often jut need to crop, resize and adjust the brightness of an image and I can usually do this in GC before Photoshop launches. Version 6 is Leopard compatible and is well worth the US$35 to get rid of the nag screen.
5. Daylite–When Address Book just isn't enough, step up to a complete contact, organization, project and task manager in Daylite (US$189). Version 3.5 syncs with Address Book, so your iPhone is always up-to-date and the beta of version 3.6 works with Leopard.
What are the first applications that you installed in Leopard?