SISC is an extensible Java based interpreter of the algorithmic language Scheme. SISC uses modern interpretation techniques, and handily outperforms all existing Java interpreters (often by more than an order of magnitude).
sisc is small and works nicely
Stable and works.
It may not be quite a 100% complete and correct implementation of R5RS, but it's *very* close. The only possible bug I've found: it may not correctly read exact rational numbers when they are entered as decimals with the "#e" notation.