Hmm... the infamous "quiver"...
I like all of my bikes to have a generally specific use:
XC HT (currently, a '97 IF Deluxe... someday to be replaced by a 29er HT)
XC FS (currently, a '98 Yeti Lawwill DH-6, planning to replace with a Jet 9 later this year)
DH/FR (currently, same Yeti, although I switch to beefier DH tires, hoping to repalce it someday...)
DS/DJ (currently, a '98 Chameleon... could use repair/upgrades right now..)
Road ('97 Colnago Master Ti. I have no plans to race road, so one bike will do for now...)
Commuter/Touring (none right now, hoping to get an I.F. Independance some day)
Cross-bike... I had one for awhile... wish I hadn't sold it, but needed money and it was the bike I rode the least. Had no real desire to race cross back then, although that may change in the future...
BMX - Mine's a 20"... I don't ride mine very often, but it's fun to have around and a nice one can be bought for stupid-cheap compared to mtn and road bikes.
Then there's the 'graveyard' of partial frames and bikes I still own but don't ride anymore and aren't really worth enough (or have sentimental value, or both) to try selling. I have... 5 of these...