Like viriods, which make a virus look huge by comparison:
> Viroids are plant pathogens that consist of a short stretch (a few hundred nucleobases) of highly complementary, circular, single-stranded RNA. Viroid genomes are extremely small in size, ranging from 246 to 467 nucleotides (nt), and consisting of fewer than 10,000 atoms. In comparison, the genome of the smallest known viruses capable of causing an infection by themselves are around 2,000 nucleobases in size. The human pathogen hepatitis D virus is similar to viroids.
> Viroid RNA does not code for any protein. Their replication mechanism uses RNA polymerase II, a host cell enzyme normally associated with synthesis of messenger RNA from DNA, which instead catalyzes "rolling circle" synthesis of new RNA using the viroid's RNA as template. Some viroids are ribozymes, having catalytic properties which allow self-cleavage and ligation of unit-size genomes from larger replication intermediates.
Source: http://en.wikipedia.org/wiki/Viroid
In case you were wondering what a ribosyme is
> A ribozyme (ribonucleic acid enzyme) is an RNA molecule that is capable of catalyzing specific biochemical reactions, similar to the action of protein enzymes.
Source: http://en.wikipedia.org/wiki/Ribozyme
The action of ribozymes led to the RNA world hypothesis, as the mechanism for how you could have a simple system from which DNA and proteins can come as later optimizations on particular aspects. Some ribozymes are able to go as far as catalyze the building of their own RNA structure in the right environments (albeit, with limited success so far).