A brief look at the Halloween Gambit
The Halloween Gambit is an aggressive but dubious piece sacrifice in the Four Knights Game, 1.e4 e5 2.Nf3 Nc6 3.Nc3 Nf6 4.Nxe5?!, which I have used about half a dozen times in casual games at the local chess club (though I would never use it in a serious tournament or match game).
White's idea is that after 4...Nxe5 White can strive to crush Black using the central pawns and force Black's knights to retreat to passive positions. To my mind, the fact that White may objectively only be slightly worse after such a continuation reflects upon how rich in possibilities chess is. If Black tries to hold onto the extra piece then White generally gets significant (though often not full) compensation for it. One popular line runs 4...Nxe5 5.d4 Nc6 6.d5 Ne5 7.f4 Ng6 8.e5 Ng8 9.d6 cxd6 10.exd6, which gives White an imposing pawn on d6.
If Black plays ...Qb6 or ...Qf6 and then plays ...Kd8 to prevent the loss of the a8-rook to Nb5-c7+, then White generally gets good compensation for the sacrificed knight, although there is a flaw in this variation (with 10...Qf6 11.Nb5 Nxf4!, sacrificing the rook on a8, Black gets very good attacking chances).
The other main accepted line of the gambit is 4...Nxe5 5.d4 Ng6 6.e5 Ng8 7.Bc4. Here the most critical response is probably Max Euwe's 7...d5 8.Bxd5 c6 (8...N8e7 is also good), where White maintains compensation for the piece but with best play Black should be slightly better.
Thus the Halloween Gambit can be successful in rapid games and is the sort of line that can provide fans of unorthodox openings with a lot of fun. However, I don't recommend it for use in serious games because Black has various ways to return the piece and reach at least an equal game, the biggest problem being Jan Pinski's suggestion 5...Nc6 6.d5 Bb4, which generally results in Black getting the majority of the attacking chances. Indeed, even 5...Bd6 6.dxe5 Bxe5 looks at least equal for Black. I have seen people compare the Halloween Gambit to the Cochrane Gambit, but this issue represents the most important difference between the two- in the Cochrane Black has no way to return the piece early in the game and come close to reaching equality, let alone a better position, and so must hold onto the piece and withstand White's attempts to squeeze Black using the central pawns.
The King's Gambit and John Shaw's book
After this bit of light entertainment, some "heavier" stuff is in order. I intend to return to the Evans Gambit soon with a look at the declined variations, but for now, I feel it is time to update my coverage of the King's Gambit, especially in view of the large book that came out on the opening by John Shaw, and to expand upon it, as I don't think I got around to covering the declined variations (notably the Falkbeer and 2...Bc5- I have faced 2...Bc5 a few times from the white side recently as well).
My general opinion of the book is favourable- it is an impressively thorough coverage, and Shaw has come up with a number of suggestions for both sides that I hadn't considered, so there is plenty to examine. He also agrees with me that the Quaade Gambit approach vs. the ...g5 lines (with an early Nf3 followed by Nc3) is probably White's best way to generate unbalanced, open-ended and equal play. However, on the downside, I think he is too dismissive of some sidelines that fall outside of his main repertoire, most notably the King's Bishop Gambit (I have used 3...Nc6 as my main antidote to 1.e4 e5 2.f4 exf4 3.Bc4 for about six years, but I feel that calling it a refutation is too strong- I think White should be able to reach equality in the 4.d4 lines and maybe 4.Nf3 also) and the piece sacrifice lines of the Rosentreter for instance.
Another important source that I expect to be using is the Chess-Brabo blog, which contains an interesting discussion on the Fischer Defence vs. the Quaade Gambit. I definitely need to revisit those lines, especially as I sometimes get them from both sides of the board (mainly White, but I've had a couple of games as Black which reached these lines via transposition also).