Most importantly of all, MySQL 5.0 supports node clustering, natively. What does this mean? You can have 3 DB servers all supporting instant fail over with each other. Stick an axe through one of your servers and your DB is still available. You can repair the broken node and bring it back up without any interruption of service throughout the entire affair.
Now that rocks.
Edit --- seems like that feature was introduced in 4.1.6(ish) and it's still only for tables in memory
Not so kick ass,
book is death - wife is stupid