I have been following the SB Nation Stampede Blue undrafted free agents for this year and there are a couple really cool prospects. Here is the link to that page and you can refresh for the most recent updates:

http://www.stampedeblue.com/2015/5/2/8537477/tracking-the-indianapolis-colts-undrafted-free-agents

The few players that stand out to me are:

Ezell Ruffin, WR, S.D. State

Junior Sylvestre, LB, Toledo

Terrell Hartsfield, DE/DT, OLB, Cincinnati

Hartsfield stands out the most, this kid is a beast. Very strong bull rush and surprisingly fast off the edge. Doesn't overrun plays too much and has a knack for where the balls at. Check out this 9 minute prospect video. I am surprised this guy didnt get drafted... And they are not playing against weak opponents. Not the SEC, but some tough games including East Carolina and Ohio State.