Happy Birthday Uncle Dave!
Happy 50th Birthday to Dave Winer, without whom my usage of the net would be completely different.
Seth wrote a very good article that describes quite well the impact Dave has got on many things (like XML-RPC, RSS, etc), so I won't come back to that. But on a more personal note, I must add that without Dave and his vision I would certainly not be making a living building online applications.
I originally went to Frontier as a hobbyist looking for more ways to scripting Mac OS (I was a sound engineer and stage manager, by then), but I got my first important job as a Frontier developer (with an English publishing company) directly from Dave and UserLand.
Obviously I worked with Macrobyte thanks to Frontier too, and even my current job I got it for the same reason: my current boss started writing his own Frontier solutions but he got rapidly too short on time to continue. So he looked for a French Frontier developer on UserLand's developers page... and I was the only hit! :)
So thank you, Dave, and happy birthday again!