Pineapple weed. It grew next to the chicken house on my grandmother's farm. Whenever I smell it I'm instantly reminded of her. She died in 1970.
There's also a smell of the combination of the ditch grass/weeds in the summer and the dust from a gravel road. Reminds me of getting the mail out of the rural mailbox as a child in the 60's. Hard to describe but when it hits me it hits me.
Some of the same others have mentioned. New mown hay, mimeographed copies.
Some other unique ones. The smell of my jukebox. Its from the heat and the effect of the heat on the light machine oil in the mechanisms. Very faint but definitely there.
The smell of the inside of my old car. Slightly musty with a bit of old gas. Sometimes I just sit in it to take in the simplicity of the dash layout, the smells, and I think about the times it has existed through and the people it has carried to different significant and insignificant life events.
Damn, I'm waxing poetic today, LOL!