This line is said by Humphrey Bogart
in the (apparently) classic film Casablanca
. His character, Rick Blaine, is talking to Ingrid Bergman's
Ilsa Lund, trying to tell her that it's okay to leave on the plane because their memories of being together in Paris
are always going to be there for them, but she'd regret not leaving with her husband.
Or something like that: I haven't seen the film. This is an iconic quote that is often repeated, often used as a clip, and almost always completely out of context since it's part of a longer line (but at least it's correct, "Play it again, Sam" is from a Marx Brothers film). The even more well known "Here's looking at you, kid" is said later in the same exchange.
"We'll always have Paris" was placed 47th on the American Film Institutes list of 100 movie quotes. There are four more from the same film. This list is largely a load of rubbish, though – it seems to be a list of lines people are supposed to think are great lines. I can think of a more memorable line from Goldfinger than "Shaken, not stirred," I'm sure anyone who has seen it can too…
I Googled this phrase and the first thing to come up in the results was a Wikipedia page about a Star Trek episode. I full text search this site and what are the first three hits? Frickin' Star Trek.