"I've yet to find one that actually uses R-Trees." That's because R-Trees don't scale well with random write loads. R-tree insertion performance is extremely

"I've yet to find one that actually uses R-Trees." That's because R-Trees don't scale well with random write loads. R-tree insertion performance is extremely dependent on insertion order (search for "sort tile recursive"). They're best used for problems where the data can be bulk-loaded and left alone. If random writes are an important part of the problem (as they are for most web-based tools), R-Trees are a bad idea.