In Star Trek: First Contact, the Borg Queen comes on to Data and asks him when the last time he was intimate and he states how long it has been for him - is it known to whom he was referring? Based on the amount of time he states, it is possible or likely that he is referring to Tasha Yar?
BORG QUEEN: Are you familiar with physical forms of pleasure?
DATA: If you are referring to sexuality, I am... fully functional, programmed in... multiple techniques.
BORG QUEEN: How long since you've used them?
DATA: Eight years, seven months, sixteen days, four minutes, twenty-two...
BORG QUEEN: Far too long.
The episode "The Naked Now" is when Data and Tasha were, ahem, "intimate", and that episode occurred on/around star date 41209.2. This corresponds to March 17th, 2364.
The movie First Contact occurred on/around star date 50893.5. This corresponds to November 23rd, 2373.
Now, if I subtract 8 years, 7 months, and 16 days from the date of First Contact, I get a date of April 7th, 2365, which is not the same as the date when Data and Tasha had relations. This corresponds to a star date of about 42264.4, meaning that Data's last usage of his "multiple techniques" was somewhere between the episodes "Where Silence has Lease" (star date 42193.6) and "Elementary, Dear Data" (star date 42286.3).
Could Data have been "practicing" his techniques all by his lonesome? I would rather not speculate, but if we're looking for an in-universe explanation then the dates suggest it can't be Tasha Yar that he is referring to, unless he's into necrophilia. ;)
The most likely explanation is an out-of-universe one, suggested in a comment by Russell Borogove:
Most likely, the First Contact script writer, working in 1995, checked that The Naked Now aired in 1987, called that eight years, and added arbitrary month, day, etc.
So, it likely is Tasha that Data is referring to (since there is no evidence of any other intimate encounters), but the script writers just got their in-universe and out-of-universe dates mixed up.
Another candidate is Lt Jenna D'Sora - but that was only 6 years before the movie (in-universe time): http://en.memory-alpha.org/wiki/In_Theory_%28episode%29 - also, it is unknown if he was 'intimate' with her in the way implied by the movie (we only see them kissing, rather unpassionately).