Considering Contract J.A.C.K. is a prequel to NOLF 2 (though released after NOLF 2) there are of course several No One Lives Forever references.
In Chapter 1: The Proposition, on the billboard across the message machine are several references. The opera poster is Inge Wagner from NOLF 1. The pink paper with 3 girls are the three assassins Cate needs to kill at the end of NOLF 1. La Troupe refers to La Troupe de Pantomine from NOLF 2, a group of mime cronies, headed by Pierre, who was hired by the Director to kill Cate Archer.
Also, the "how to pick up classy chicks by Steven Lee" likely refers to Steve Lee, a Senior 2D/3D Artist of Contract J.A.C.K.
There are some Wanted posters hanging around for Cate Archer also. We've noticed them in Chapter 2: Trial by Gunfire, and Chapter 7: Il Pazzo.
And Cate Archer even makes a quick cameo in person during the opening scene of Chapter 3: Welcome to Czechoslovakia.
Strange creature in the moon base
In the battery recharger room of the moon base, you can hear something snarling and giggling and it appears he occasionally also says "Gollum, Gollum". There is a panel in the corner under two arrow shapes that can be activated, revealing this strange creature in a hidden room. It appears Gollum from Lord of the Rings made it into the game though the resemblance is rather questionable.