Image by Keith Allison via Flickr
In case you're stumped, here they are:
1. Hit
2. Walk
3. Hit by Pitch
4. Fielder's Choice
5. Reached on Error
6. Dropped Third Strike
7. Catcher's Interference
8. Fielder's Interference
Now, you will find people who argue additional ways exist, but they are almost always variations of one of the 8 listed above. For example, yes, a batter can reach first on a wild pitch with 3 balls already in the count, but folks, that's still a walk, covered in #2.
And for you wise-acres who want to add "Being inserted as a pinch-runner," clever but wrong. The list is ways a batter can reach first, and a pinch-runner ain't a batter!
There also happen to be exactly 8 days until Opening Day 2010...can't wait!