I've got a couple surprises that I can think of:
Alex Ramirez Like a few others, this was my first experience sending to Japan (and this was a good six or seven years ago), so my hopes weren't too high (even though I had heard the unknown A-Ram was a good signer). I threw a couple cards in there as well as a few dollars for his time and postage. Two and a half years later, it found its way back to America, with two cards signed and my money returned to me.
Bill Mazeroski This was another one of my early ones, and I only tried it because I had come across an address for Maz in Greensburg (ten minutes from my hometown). So I sent it out on a whim, not knowing what his signing habits were through the mail (being a HOFer and all). About three weeks later, I got one back signed (postmarked in Florida, no less), and I soon thereafter discovered that Maz doesn't sign too well through the mail. I guess being a local boy helped.
Omar Olivares I was surprised not only that I got my card signed (it was my first attempt at Puerto Rico), but I was surprised at what I received. I remember sending two cards (an 01 Topps and an 02 Topps), and about a month later, I got my 01 Topps signed with a page-long letter from MRS. Olivares talking about her family ties to the area I lived in at the time. I still have the letter - it's one of the oddball mementos I hold onto.