You don't say what you tried to install, but
Inkscape is the only open-source alternative to Illustrator that's worth the comparison (albeit there isn't much that can compete with Illustrator at any price), so I'll assume that's what it was.
If the packaged version doesn't yet support Leopard, you could try building it yourself via
Fink, which - in theory - should sort it out (I haven't tried it, though, so can't guarantee success).
Alternatively, it isn't free, but
Intaglio is pretty reasonable and not too expensive or, at a higher price but still cheaper than Illustrator, there's
Create. Both have time-limited demo versions available.