My youngest has always loved blocks. We used to have those clear Peek-a-boo blocks and the other kids would build towers for her and she would knock them down at 5 months. By 9 months she was building her own towers. Around 1 she started playing with Mega Blocks. She's going to be 3 next month and graduated to the medium sized building blocks at around 2 1/2 and loves to play with her brother's Legos. Her favorite, though, is her plain wooden blocks. She builds all sorts of crazy castles and likes to stack them to see how high she can get her tower before it falls.
Yes, she didn't start putting them together herself until around 6 months I think, but she still loved playing with them. She still plays with them all the time at 19 months, and they were great for helping teach her colors and counting.
I think my son started playing with mega blocks around 6 months, he actually started putting them together at about 9 months. He started playing with wooden blocks about the same time and actually started stacking them at about a year old.