user7773: [Update:] It's called "Tan gambit" after an Alekhine game. Analyzed below. White should dicline the pawn, since. It can be played as a drawing line for black (no queens).

d4 d5 2. c4 c6 3. cxd5 Nf6!? 

This seem to me a good gambit line. I think it's a good way of playing but no one does according to www.365chess.com Some people find exchange variation boring. What is the reason that no one is playing it? 
Take a look at the position after 1. d4 d5 2. c4 c6 3. cxd5 Nf6 4. dxc6 Nxc6 5. Nf3 Bg4 6. e3 e5. Black is very active.
Of course white can avoid it by 3.Nf3. Meaning that slav players still have to know the exchange variation. A gambit does not need to be 100% correct. But a gambit no one is playing? 
(I found it via 1.d4 d5 2. c4 Nf6. With no one I don't mean 0).
[FEN ""]

1. d4 d5 2. c4 c6 3. cxd5 Nf6!? (3. cxd5 cxd5) (3. cxd5 Nf6 4. dxc6 Nxc6 5. Nf3 Bg4 6. e3 e5)

[Update]: I used lichess (poor) analysis feature to analyze some games. [Between strong players].

Nf3 Bg4. [What I would play].

7. Be2 [What I would be most afraid of]. ½-½, Reinhold (2139) vs. Stevermueer (2150)
7. Bb5 [Messy internet game] 0-1, Drozdovskij (2509) vs. Dobrov (2515)

Nf3 e5. [Queens are exchanged].

6. dxe5 1-0, Sandipan (2637) vs. Elorta (2363)
6. Nxe5 ½-½, Vorobiov (2577) vs. Zvjaginsev (2646)

Nf3 Bf5.

5. Nf3 Bf5 1-0, Alekhine vs Tan

user8687: Black is active, but he's hardly "sac-a-pawn" active. Once white finishes his development and places his rooks and pieces on the open files and lines, I see no way for Black to stop white from exchanging pieces to a safe pawn up endgame. If I'm giving up a pawn like this, I want activity and something dynamic, like the ability to keep my opponent's King from castling. I just don't see enough from this pawn sac to see it as any good. Maybe in blitz once or twice. But I would hate to have to sit through a classical time control game with this position. 

user4362: In the chessok opening tree there are 28 games with this gambit. Going by the computer evaluation it seems to be playable, but the score is atrocious (>80% for white).
So here you have a gambit that is pretty unknown, scores horribly and can easily be ignored if white is not in a mood for adventures. Also it doesn't seem to lead to dangerous complications as far as I can see. There is no automatic attack on the white king, in the worst case white just returns the pawn. 
All in all not very surprising that it is rarely played. 

user2299: If you want to play this gambit, the best way to get there is by playing 2...Nf6!? 3.cxd5 c6!. White is almost certain to play 3.cxd5 which is both the natural move and the theoretical refutation of Nf6. Of course he can still ignore your pawn on c6 and transpose to the Exchange Slav, but at least with this move order it's not a sure thing that he wanted to play the exchange variation.
The gambit 1.d4 d5 2.c4 Nf6 3.cxd5 c6 was played in the game Udovcic–Canal, Venice 1953. (Thanks to @r4 for the link.)

7773: <p>[Update:] It's called "Tan gambit" after an <a href="http://www.chessgames.com/perl/chessgame?gid=1013025" rel="nofollow">Alekhine game</a>. Analyzed <a href="http://en.lichess.org/YHUfBqRM#9" rel="nofollow">below</a>. White should dicline the pawn, since. It can be played as a drawing line for black (no queens).</p>

<ol>
<li>d4 d5 2. c4 c6 3. cxd5 Nf6!? </li>
</ol>

<p>This seem to me a good gambit line. I think it's a good way of playing <a href="http://www.365chess.com/opening.php?m=6&amp;n=43&amp;ms=d4.d5.c4.c6.cxd5&amp;ns=7.8.23.36.43" rel="nofollow">but no one does according to www.365chess.com</a> Some people find exchange variation boring. What is the reason that no one is playing it? </p>

<p>Take a look at the position after 1. d4 d5 2. c4 c6 3. cxd5 Nf6 4. dxc6 Nxc6 5. Nf3 Bg4 6. e3 e5. Black is very active.</p>

<p>Of course white can avoid it by 3.Nf3. Meaning that slav players still have to know the exchange variation. A gambit does not need to be 100% correct. But a gambit no one is playing? </p>

<p>(I found it via 1.d4 d5 2. c4 Nf6. With no one I don't mean 0).</p>

<pre><code>[FEN ""]

1. d4 d5 2. c4 c6 3. cxd5 Nf6!? (3. cxd5 cxd5) (3. cxd5 Nf6 4. dxc6 Nxc6 5. Nf3 Bg4 6. e3 e5)
</code></pre>

<p>[Update]: I used lichess (poor) analysis feature to analyze some games. [Between strong players].</p>

<ol start="5">
<li>Nf3 Bg4. [What I would play].</li>
</ol>

<p><a href="http://en.lichess.org/vKcI2pji#13" rel="nofollow">7. Be2 [What I would be most afraid of].</a> ½-½, Reinhold (2139) vs. Stevermueer (2150)</p>

<p><a href="http://en.lichess.org/TEi8oCJz#19" rel="nofollow">7. Bb5 [Messy internet game]</a> 0-1, Drozdovskij (2509) vs. Dobrov (2515)</p>

<ol start="5">
<li>Nf3 e5. [Queens are exchanged].</li>
</ol>

<p><a href="http://en.lichess.org/LwHQwqWy#11" rel="nofollow">6. dxe5</a> 1-0, Sandipan (2637) vs. Elorta (2363)</p>

<p><a href="http://en.lichess.org/omn1OnFv#11" rel="nofollow">6. Nxe5</a> ½-½, Vorobiov (2577) vs. Zvjaginsev (2646)</p>

<ol start="5">
<li>Nf3 Bf5.</li>
</ol>

<p><a href="http://en.lichess.org/YHUfBqRM#9" rel="nofollow">5. Nf3 Bf5</a> 1-0, Alekhine vs Tan</p>
 8687: <p>Black is active, but he's hardly "sac-a-pawn" active. Once white finishes his development and places his rooks and pieces on the open files and lines, I see no way for Black to stop white from exchanging pieces to a safe pawn up endgame. If I'm giving up a pawn like this, I want activity and something dynamic, like the ability to keep my opponent's King from castling. I just don't see enough from this pawn sac to see it as any good. Maybe in blitz once or twice. But I would hate to have to sit through a classical time control game with this position. </p>
 4362: <p>In the <a href="http://chessok.com/?page_id=352">chessok opening tree</a> there are 28 games with this gambit. Going by the computer evaluation it seems to be playable, but the score is atrocious (>80% for white).</p>

<p>So here you have a gambit that is pretty unknown, scores horribly and can easily be ignored if white is not in a mood for adventures. Also it doesn't seem to lead to dangerous complications as far as I can see. There is no automatic attack on the white king, in the worst case white just returns the pawn. </p>

<p>All in all not very surprising that it is rarely played. </p>
 2299: <p>If you want to play this gambit, the best way to get there is by playing 2...Nf6!? 3.cxd5 c6!. White is almost certain to play 3.cxd5 which is both the natural move and the theoretical refutation of Nf6. Of course he can still ignore your pawn on c6 and transpose to the Exchange Slav, but at least with this move order it's not a sure thing that he wanted to play the exchange variation.</p>

<p>The gambit 1.d4 d5 2.c4 Nf6 3.cxd5 c6 was played in the game <a href="http://www.365chess.com/view_game.php?g=2518171" rel="nofollow">Udovcic–Canal, Venice 1953</a>. (Thanks to @r4 for the link.)</p>
