“Same Old Lang Syne” by Dan Fogelberg. Walloon’s post above links to the lyrics.
I’m surprised that no one has mentioned Dan Folgerberg’s Same Old Lang Syne… 
“Out of the Snow” by the Amazing Rhythm Aces:
“And I never expected to see you again
Must a been seven years ago
Now that you’re here, why don’t you come in
Don’t stand outside in the snow”
Full lyrics here.
“Elderly Woman Behind the Counter in a Small Town” by Pearl Jam.
Teddy Pendergrass, owner of the best baritone ever, did **“She’s Over Me” **
Great song, about running into an old love, and how he realized she was gone.
*And I looked in her eyes,
and I saw enough to realize
it’s dead and gone,
i don’t have a prayer of getting her back where she used to be,
she’s over me.
I’ll spend my whole life knowing, thinking of how good we were
She’s over me, but I’ll never be over her.*
West LA Fadeaway, Grateful Dead.
*I met an old mistake walking down the street today
I met an old mistake walking down the street today
I didn’t want to be mean about it
I couldn’t think of one good thing to say.
*
Big Head Todd And The Monsters - “Wearing Only Flowers” from Strategem. The relevant lyric (from memory, but I think I’ve got it right) is:
“After ten years taken
Accidentally meeting
Realized we were mistaken…”
…and then goes on to mention getting married after breakfast.
Big Head Todd And The Monsters - “Wearing Only Flowers” from Strategem. The relevant lyric (from memory, but I think I’ve got it right) is:
“After ten years taken
Accidentally meeting
Realized we were mistaken…”
…and then goes onto mention getting married after breakfast.
What was the song by Folgelberg?
Another entry: How Much I Feel by Ambrosia.
How’s your life been goin’ on?
I got a wife now, years we been growin’ strong
There’s just something that I’ve got to say
Sometimes when we make love, I still can see your face
“How did we get from saying ‘I love you’” by Great Big Sea
“Reunited” by Peaches and Herb. (I think)
Anyway hasn’t been said yet, you people are slip’n! 
First thing I thought of was “And still”, sung by Reba McEntire.
“I Can’t Help It If I’m Still in Love With You” by Hank Williams (Sr.)
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…plus isn’t there some song with a holiday theme? Something about Christmas Eve?
“CBGB’s” by Syd Straw.
Do phone calls from old lovers count? 'Cause then there’s also:
“Diamonds and Rust” by Joan Baez (which appears to be about a forlorn phone call from Bob Dylan).
(Can you tell I live for these “name a song about ________” threads?)
How about an unexpected letter from an old lover?
“Once in a Very Blue Moon” by Nanci Griffith
“I’m Carrying” by Paul McCartney is about meeting an old girlfriend. This song is so not about Linda, I always wondered what she thought about it.
“Hello Like Before,” by Bill Withers.
I’m telling you, folks…you really need to listen to more Bill Withers. The man is a very, very talented singer and he gets nowhere near the credit he deserves.
“Queen Anne’s Lace” by The Del McCoury Band
and (sorta)
"First Episode at Hienton by Elton John/Bernie Taupin.
What was that song about seeing your ex-lover in a grocery store? Wasn’t it “Rio” by Duran Duran?