My lonely lens would have to be the Tamron 28-200. It travels with me to all my photoshoots and almost never gets to leave the Pelican case. The Tamron 28-105, FA50 and DA*16-50 get all the attention. With the new FA77 and soon to arrive DA40, it will probably start to cry.
Second most lonely is a Promaster 100mm Macro (Manual Focus) that I bought for table top miniatures photography (scale models, typically between 25-60 mm tall). Excellent for that purpose, unused for everything else.
Then there is the forgotten trio - 3 Pentax branded zooms that don't get taken out. The 17-35 (IIRC) that came as a kit lens with the *istD, an old 28-80 and an 80-200. The 28-80 was bought used as an "it can't hurt to have a cheap backup" lens, but it isn't as well made as the Tammy 28-105. The 80-200 was new, but long unsold at the camera store, so it was too cheap to pass up, but not really needed. The 17-35 convinces me how good the 16-50 is every time I compare the pictures (lots of distortion with the 17-35).