Strong's #5068: nadab (pronounced naw-dab')
 a primitive root; to impel; hence, to volunteer (as a soldier), to present spontaneously:--offer freely, be (give, make, offer self) willing(-ly).
Brown-Driver-Briggs Hebrew Lexicon:
 
  nâdab 
 
 1) to incite, impel, make willing
 
 1a) (Qal) to incite, impel
 1b) (Hithpael)
 
 1b1) to volunteer
 1b2) to offer free-will offerings
 
  Part of Speech: verb
Relation: a primitive root
Usage:
This word is used 17 times:
Exodus 25:2: "of every man that  giveth it willingly with his heart ye shall take"
Exodus 35:21: "and every one whom his spirit  made willing, and they brought the LORD's"
Exodus 35:29: "and woman, whose heart  made them willing to bring for all manner of work,"
Judges 5:2: "for the avenging of Israel, when the people  willingly offered themselves."
Judges 5:9: "My heart is toward the governors of Israel,  that offered themselves willingly among the people. Bless ye the LORD."
1 Chronicles 29:5: "to be made by the hands of artificers. And who  then is willing to consecrate his service this day unto the LORD?"
1 Chronicles 29:6: "with the rulers of the king's work,  offered willingly,"
1 Chronicles 29:9: "Then the people rejoiced, for  that they offered willingly, because with perfect heart they offered willingly"
1 Chronicles 29:9: "because with perfect heart  they offered willingly to the LORD: and David the king also"
1 Chronicles 29:14: "is my people, that we should be able  to offer so willingly after this sort? for all things"
1 Chronicles 29:17: "As for me, in the uprightness of mine heart  I have willingly offered all these things: and now have I seen"
1 Chronicles 29:17: "thy people, which are present here,  to offer willingly"
2 Chronicles 17:16: "And next him was Amasiah the son of Zichri,  who willingly offered himself unto the LORD; and with him two hundred"
Ezra 1:6: "and with beasts, and with precious things, beside all  that was willingly offered."
Ezra 2:68: "to the house of the LORD which is at Jerusalem,  offered freely for the house of God to set it up"
Ezra 3:5: "of the LORD that were consecrated, and of every one  that willingly offered a freewill offering unto the LORD."
Nehemiah 11:2: "blessed all the men,  that willingly offered themselves to dwell at Jerusalem."