I can imagine it looking great in the ground as a centerpiece to the garden or on a curve of a flower bed, somewhere that it won't look like a lost item, but distinctly stands out in your garden. Somewhere along the edge that people may walk around you garden would be good, easy to spot and admire.
Also, if it fills with rain water, perhaps leave it and see what wildlife comes to, or dump it if fear for plants or nesting mosquitoes that lay eggs and grow as larvae in standing water. Either way, with or without water, this piece would look great in your garden facing outward towards the people that walk past it, so it can be admired. Maybe you could accent it with some smaller, similar-in-color stones. Just a few thoughts!
It may have functioned as the reservoir portion of a fountain and had more parts to it originally. It could be made into a fountain by using a submersible water pump with a bird bath or something inside of it. Here is a picture of a larger fountain that would be similar to what I was thinking. http://www.google.com/search?q=fount...Zwp7DB_UFKdXM: