Dell Software's Toad Query Optimizer (for Oracle and SQL Server) has an engine that does exactly that. It takes your SQL, tries rewriting it a bunch of different ways, throws in different compiler hints, and measures the response time/reads/CPU/etc for each execution, then gives you a set of graphs and recommendations.

I wanna say it was around $1500/seat last time I looked at it. It's worth every penny if you have to write a lot of complex queries. I've bought it for just one senior member of a dev team, and everybody brings their worst queries to that person for tuning.

Here's the trick, though: after it optimizes a query for you, keep your original query around. You're not going to want to try to edit the query that Toad produces - it can be horrifically complex. Instead, edit your original query, then run it through Toad again.