There are rarely any NBA players who are not drafted who make an impact in the NBA. Ben Wallace, Bruce Bowen, Raja Bell, Avery Johnson, and Brad Miller were not drafted but have been solid players for their respective teams--some as role players, some as All-Stars.
I am going to look at the top players not drafted in this years draft.
Drew Neitzel PG Michigan St - He was recently signed to play in the summer league with the Minnesota Timberwolves, but was one of the top players in college not drafted.
He is like a Gerry McNamara of Syracuse where he is too small and unathletic to be an NBA point guard. He is a perfect overseas player. He would be lucky to be on a roster by the fall.
Davon Jefferson SF USC - After he declared for the draft he was expected to be a first round pick. After workouts his stock had dropped as many scouts suggested he should have returned to college. For some reason he stayed in and is now looking for a job.
He is a reason why David Stern wants to make a 20- year-old age limit.
Jamont Gordon G Miss St - The best player not drafted by far. He has all the tools to be an effective NBA player. He has a chance to be one guy who defiantly will make GM's angry they did not draft him.
Expect to see him make an NBA roster and eventually grow into a rotation player.
Brian Butch C Wisconsin - Coming out of high school with LeBron James' class he was expected to be a lottery pick. Due to injuries over his career he has dropped out to most likely become an overseas player. If he can stay healthy consistently he will be on an NBA roster in the near future.
Pat Calathes SF St. Joes - He is a 6'10" point guard basically who was considered a second round pick all this past season. He will get a shot in some summer camp and expect him to sign with a team.
Josh Duncan PF/C Xavier - He shut down Greg Oden when they met in the 2007 near upset in the NCAA tournament. He was the strongest player in the Orlando pre-draft camp also. He does not wow you in any way but just gets boards, sets screens, and can knock down a mid range jumper.
Expect him to sign with a team and work in the D-League.
Gary Forbes SF UMASS - He was all the buzz around Orlando pre-draft camp. He was really impressive and many scouts talked highly of him. I was surprised to not see him get drafted, but expect to see him on a summer league team and eventually find a spot on an NBA roster. Can develop a bit in the D-League.
Chris Lofton PG/SG - The best three point shooter in college basketball should have at least been looked at. He can be a solid player coming off the bench. Expect him to impress in a summer camp and find himself on an NBA roster. He can create his own shot and be a solid player at the next level.
David Padgett PF/C - A couple of years ago he had top five pick potential. He had injuries but showed dominance when he played. I was surprised to not see him get much talk from scouts. I expect him to work hard and sneak on to the radar in a summer camp. He can be an energizer player off the bench.
Will Daniels SF URI - At one point during the season he was considered a first round pick. I do not know what happened to him. Played against him and he has an NBA built body who can score in so many different ways. He has all the tools to find himself on an NBA roster.
DeMarcus Nelson PG - A solid pure Duke PG who will go overseas. Had his peak a couple of seasons ago. I would not be surprised if he gets signed by a team because he is a very smart basketball player and does many things right.