The Best Free Engines

According to CCRL’s 40/40 rating list the following engines are the highest rated free and open source engines. Their results against Houdini 3, are listed here. Not all of them played against Houdini 4, so H3 is the engine I chose for the benchmark:

3-Stockfish 4 64-bit 4CPU (vs. Houdini 3 = +14 -13 =53)
4-Critter 1.6a 64-bit CPU (vs. Houdini 3 = +16 -47 =92)
6-Bouquet 1.5 64-bit 4CPU (vs. Houdini 3= +3 -16 =15)
7-Gull 2.3 64-bit 4CPU (vs. Houdini 3 = +2 -18 =37)
8-Strelka 5.5 64-bit (vs. Houdini 3 = +1 -19 =19)
9-Hannibal 1.4b 64-bit 4CPU (vs. Houdini 3 = +1 -15 =14)
10-Protector 1.5.0 64-bit 4 CPU (vs. Houdini 3= +2 -18 =10)

Komodo 5 scores against various engines:
Houdini 3 = +11 -17 =48
Stockfish 4 64-bit 4CPU = +12 -16 =32
Critter 1.6a 64-bit CPU 3 = +8-2 =14
Bouquet 1.5 64-bit 4CPU = +11 -3 =16
Gull 2.3 64-bit 4CPU = +14 -1 =16
Strelka 5.5 64-bit = +0 -0 =0
Hannibal 1.4b 64-bit 4CPU +0 -0 =0
Protector 1.5.0 64-bit 4 CPU = +14 -0 =16

I think this shows that at slow time controls Stockfish 4 is the best engine you can get without spending any money. Used with Arena or SCID, there appears no reason for most of us to purchase any commercial programs.  You can also download tons of games to make your own database and various opening books are also available.