Obscure references you got and had sort of a "nerdgasm" over

[I spent the winter writing songs about getting better]

Obviously this only counts if unexpected. If it's an expected place (like obscure sci-fi references on a science fiction forum) then it's not quite the same.

Too that stood out to me and made me think at the time "OMG POSTER YOU ARE THE COOLEST PERSON EVER!"

-The person who simply posted the entire lyrics to "The Content of Lincoln's Pockets" on the AV Club's comments for Lincoln.
-The time I saw someone quote Jawbreaker lyrics in the comments on a CNN article.

[Atlas Has Shrugged]

Any mention of Bob Dylan usually sets me off.

[DC Al Fine]

Proper use of accounting terminology.

[Napoleon]

I met a girl a few weeks ago that could cite literally any date or random factoid re: 90s movies. HOT

[kcguy]

I remember being pleasantly surprised when "Buffy tVS" made a "nobody expects the Spanish Inquisition" joke.  I wouldn't have thought that was their target audience.

"Daria" had a quite few of these.  Who'd think that a cartoon on MTV would make jokes about Machiavelli and Ayn Rand?  (A quote:  "What do you call those people from the '60's who dress funny and talk about peace and love?"  "Trekkies.")

Usually, I produce the opposite reaction.  I once made a comment to my friends about Terry Pratchett's Seamstresses' Guild, and all I got was a roomful of blank stares.  (Sadly, I also received some blank stares to mentions of Geraldine Ferraro.)

[Starbucks Union Thug HokeyPuck]

Today I realized that the "floating Poe" sound effect from The Legend Of Zelda: Ocarina Of Time, was reused for the X-parasites in Metroid Fusion for the GBA.  Definitely a WHOA moment there.