SheepShaver
Damien found SheepShaver last February, and at the time this PowerPC emulation environment (great for running Classic on Intel Macs) had 'experimental' attached to its explanation. While I don't know if it can be referred to as a solid powerhouse environment even still, it seems to have come quite a ways since February; up to version 2.3, to be exact. The latest version has support for up to 1 GB of memory, many bug and crash fixes, clipboard updates, performance enhancements and more.
While it still might not be pretty, SheepShaver is one of the few, if only, methods I've heard of for running Classic on an Intel Mac.
