Recently a lot of Monaco——and I still love it.

Adobe Garamond
Adobe Caslon
Akzidenz Grotesk
Ed
Eureka
Neutraface
Sabon
Unit

I assume we're leaving out fonts bundled with the system or Adobe CS?

The following are always installed for me and I've used each more than a few times:

Amplitude (Amp Wide makes an excellent System font and default for Mail and iChat.)
FF Legato
MVB Verdigris
Vista Sans
Knockout
FF Olsen
PMN Caecilia
FF Maiola
Freight (suite)
Imbiss (suite)
Poppi
FF Dingbats

Varies a bit, but a typical set at the moment looks like this (various clients and products). Some of them were given, i.e. I had no say in choosing them.

Freight sans + text
Verdana
Neo Tech (Alt)
ITC Kabel
Cocon
Xxxxx Script (client's own customized font)
Helvetica Neue
Gill Sans
Frutiger
Syntax
Myriad Pro
Trajan

mvb verdigris
neue helvetica
adobe garamond
univers (mostly condensed weights)

that's about it. i use plenty else, but this is what i use every day.

I am too typographically moody to have a set that is always loaded. That said, I do use Gotham on some internal stuff so it is usually open.

(in no particular order)

ITC Tiepolo
Optima
Palatino
Albertus
Hiroshige
PopplLaudatio
ITC Eras
Lithos

(in alphabetical order)

Adobe Garamond
Bembo Book
Caflisch Script
Castellar
Dante
Diotima and Ariadne
Frutiger
Gill Sans
Neuland
Penumbra
Post Antiqua (usually I set this in caps only)
Syntax

I'm with Tiffany, I go into long swoons with fonts and then deep six them for awhile. It really depends on what I'm working on.

I tend to keep Gotham open, tho, along with Thesis and Today Sans and for serifs my fallbacks lately are Adobe Garamond and Filosofia.

But I've got a bunch of stuff open right now from Tribute to Meta to Klavika, Caecilia, Requiem, Jigsaw, Dalliance, Abadi, and some weird agitated scripts I was looking at for a book cover.